Optical system and image display apparatus

ABSTRACT

An optical system which is compact, lightweight and satisfactorily corrected for aberrations and may be suitably used as an imaging optical system or an ocular optical system for a head- or face-mounted image display apparatus which forms no intermediate image. The optical system includes an optical member (7) and a diffraction optical element (8), which are decentered with respect to each other. The optical member (7) has at least three adjacent optical surfaces, at least one of which is a curved surface. At least two reflections take place between the optical surfaces. The space between the optical surfaces is filled with a medium having a refractive index larger than 1. Light rays emitted from an image display device (6) enter the optical member (7) through a first transmitting surface (5) disposed to face the image display device (6) and are reflected by a first reflecting surface (3). The reflected light rays are reflected by a second reflecting surface (4) and led to an observer&#39;s eyeball through a second transmitting surface (3).

BACKGROUND OF THE INVENTION

The present invention relates to a compact imaging optical system andocular optical system which have favorable aberration correctionperformance, a wide field angle and high resolution. The presentinvention also relates to an image display apparatus which uses theocular optical system.

In recent years, helmet- and goggle-type head- or face-mounted imagedisplay apparatuses have been developed for virtual reality or for thepurpose of enabling the user to enjoy a wide-screen image personally.

For example, Japanese Patent Application Laid-Open (KOKAI) No. 2-297516discloses an image display apparatus including, as shown in FIG. 48, atwo-dimensional display device 21 for displaying an image, an objectivecollimator lens 22, and a parallel transparent plate 23 having off-axisparabolic mirrors at both ends thereof. Display light emanating from thetwo-dimensional display device 21 is formed into parallel rays throughthe objective collimator lens 22. Thereafter, the light rays aresuccessively subjected to first transmission by one of the parallelsurfaces of the parallel transparent plate 23, reflection by the firstparabolic mirror, some total reflections in the parallel transparentplate 23, reflection by the second parabolic mirror and secondtransmission by the other of the parallel surfaces (a total of 8reflections and a total of 2 transmissions), thereby forming anintermediate image at the point F and projecting the intermediate imageinto an observer's eyeball 24.

U.S. Pat. No. 4,026,641 discloses an image display apparatus in which,as shown in FIG. 49, an object image displayed by an image displaydevice 25 is converted into a curved object image by a transfer opticalelement 26, and the object image is projected into an observer's eyeballby a toric reflecting surface 27.

In head-mounted image display apparatuses, it is important to lead animage of an image display device to an observer's eyeball without usinga relay optical system and without forming an intermediate image in theoptical path, as shown for example in Japanese Patent ApplicationLaid-Open (KOKAI) No. 6-308424.

However, in Japanese Patent Application Laid-Open (KOKAI) No. 6-308424,the optical path is formed with a half-mirror interposed therein.Therefore, there is a loss of light quantity, and it is not easy toperform bright display.

European Patent No. 0,583,116A2 discloses a head-mounted image displayapparatus which is capable of displaying a bright image without using ahalf-mirror. In this case, however, a relay optical system is used,which causes the size of the apparatus to increase unfavorably.

In an image display apparatus of the type in which an image of an imagedisplay device is relayed, as shown in FIG. 48, a relay optical systemis needed in addition to an ocular optical system. Consequently, theentire optical system increases in both size and weight, and an amountby which the optical system projects from the observer's face or headalso increases. Therefore, this type of image display apparatus is notsuitable for use as a head- or face-mounted image display apparatus.

In the optical system that focuses parallel rays to form an intermediateimage, and also in the optical system that projects an intermediateimage into an observer's eyeball, only the parabolic mirror has power.Therefore, exceedingly large aberrations are produced in these opticalsystems.

When a concave mirror alone is used as an ocular optical system as shownin FIG. 49, even if the concave mirror is a toric surface as in the caseof FIG. 49, the ocular optical system produces exceedingly largeaberrations, causing the image quality to be degraded. Accordingly, itis necessary to use a transfer optical element 26 such as a fiber platefor correcting field curvature produced by the ocular optical system.However, comatic and other aberrations cannot satisfactorily becorrected even if the transfer optical element 26 and the toricreflecting surface 27 are used.

SUMMARY OF THE INVENTION

In view of the above-described problems of the conventional techniques,an object of the present invention is to provide an optical system ofthe type having at least three surfaces and in which a space formed bythe at least three surfaces is filled with a medium having a refractiveindex larger than 1. The optical system according to the presentinvention is compact, lightweight and satisfactorily corrected foraberrations and may be suitably used as an imaging optical system or anocular optical system and also for an image display apparatus which usesthe ocular optical system.

To attain the above-described object, the present invention provides anoptical system having an optical member and a diffraction opticalelement which is adjacent to the optical member. The optical member hasat least three adjacent optical surfaces which are decentered withrespect to each other. At least one of the three optical surfaces is acurved surface, and the three optical surfaces are arranged such that atleast two reflections take place between them.

In addition, the present invention provides an image display apparatushaving an image display device for displaying an image, an ocularoptical system for leading the image displayed by the image displaydevice to an observer's eyeball without forming an intermediate realimage, and a device for retaining both the image display device and theocular optical system on an observer's head or face. The ocular opticalsystem includes an optical member having a first surface disposed toface the image display device, a second surface disposed on anobserver's visual axis to face an observer's pupil at a tilt to theobserver's visual axis, and a third surface disposed on the observer'svisual axis between the second surface and the observer's pupil. Thesecond surface is a reflecting surface. Light rays emitted from theimage display device enter the optical member through the first surface,and the light rays are reflected by the second surface and led to theobserver's eyeball through the third surface. In addition, a correctionoptical element is disposed at a position between the image displaydevice and the observer's pupil. The correction optical element producesaberrations which are opposite in sign to aberrations produced by thetransmitting surfaces of the optical member.

In addition, the present invention provides an image display apparatushaving an image display device for displaying an image, an ocularoptical system for leading the image displayed by the image displaydevice to an observer's eyeball without forming an intermediate realimage, and a device for retaining both the image display device and theocular optical system on an observer's head or face. A light-blockingmember is provided between the ocular optical system and the observer'seyeball.

In addition, the present invention provides an image display apparatushaving an image display device for displaying an image, an ocularoptical system for leading the image displayed by the image displaydevice to an observer's eyeball without forming an intermediate realimage, and a device for retaining both the image display device and theocular optical system on an observer's head or face. A numericalaperture limiting member is disposed between the image display deviceand the ocular optical system to limit the numerical aperture of abundle of light rays emitted from the image display device.

In addition, the present invention provides an image display apparatushaving an image display device for displaying an image, an ocularoptical system for leading the image displayed by the image displaydevice to an observer's eyeball without forming an intermediate realimage, and a device for retaining both the image display device and theocular optical system on an observer's head or face. The image displaydevice is a transmission type liquid crystal display device. Thetransmission type liquid crystal display device uses light limited inthe numerical aperture as illuminating light therefor.

In addition, the present invention provides an image display apparatushaving an image display device for displaying an image, an ocularoptical system for leading the image displayed by the image displaydevice to an observer's eyeball without forming an intermediate realimage, and a device for retaining both the image display device and theocular optical system on an observer's head or face. The image displaydevice is a transmission type liquid crystal display device having anilluminating device at the back of it. Assuming that the size of theilluminating device is Sb, the distance d between the illuminatingdevice and the image display device satisfies the following condition:

    Sb>d>1 mm                                                  (24)

In addition, the present invention provides an image display apparatushaving an image display device for displaying an image, an ocularoptical system for leading the image displayed by the image displaydevice to an observer's eyeball without forming an intermediate realimage, and a device for retaining both the image display device and theocular optical system on an observer's head or face. The image displaydevice is a transmission type liquid crystal display device having anilluminating device at the back of it. The illuminating device and theimage display device are tilted relative to each other such that theilluminating device and the image display device diverge from each otherat ends thereof which are remote from the observer's eyeball.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

The invention accordingly comprises the features of construction,combinations of elements, and arrangement of parts which will beexemplified in the construction hereinafter set forth, and the scope ofthe invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an image display apparatus for a singleeye which uses an optical system according to Example 1 of the presentinvention.

FIG. 2 is a sectional view of an image display apparatus for a singleeye which uses an optical system according to Example 2 of the presentinvention.

FIG. 3 is a sectional view of an image display apparatus for a singleeye which uses an optical system according to Example 3 of the presentinvention.

FIG. 4 is a sectional view of an image display apparatus for a singleeye which uses an optical system according to Example 4 of the presentinvention.

FIG. 5 is a sectional view of an image display apparatus for a singleeye which uses an optical system according to Example 5 of the presentinvention.

FIG. 6 is a sectional view of an image display apparatus for a singleeye which uses an optical system according to Example 6 of the presentinvention.

FIG. 7 is a sectional view of an image display apparatus for a singleeye which uses an optical system according to Example 7 of the presentinvention.

FIG. 8 is a sectional view of an image display apparatus for a singleeye which uses an optical system according to Example 8 of the presentinvention.

FIG. 9 is a sectional view of an image display apparatus for a singleeye which uses an optical system according to Example 9 of the presentinvention.

FIG. 10 is a sectional view of an image display apparatus for a singleeye which uses an optical system according to Example 10 of the presentinvention.

FIG. 11 is a sectional view of an image display apparatus for a singleeye which uses an optical system according to Example 11 of the presentinvention.

FIG. 12 is a sectional view of an image display apparatus for a singleeye which uses an optical system according to Example 12 of the presentinvention.

FIG. 13 is a sectional view of an image display apparatus for a singleeye which uses an optical system according to Example 13 of the presentinvention.

FIG. 14 is a sectional view of one example of an optical member to whichthe present invention can be applied.

FIG. 15 is a sectional view of another example of an optical member towhich the present invention can be applied.

FIG. 16 is a sectional view of still another example of an opticalmember to which the present invention can be applied.

FIG. 17 is a sectional view of a further example of an optical member towhich the present invention can be applied.

FIG. 18 is a sectional view of a still further example of an opticalmember to which the present invention can be applied.

FIG. 19 is a sectional view of a still further example of an opticalmember to which the present invention can be applied.

FIG. 20 is a sectional view of a still further example of an opticalmember to which the present invention can be applied.

FIG. 21 shows the principle of refraction to explain a diffractionoptical element used in the present invention.

FIG. 22 shows the principle of diffraction to explain a diffractionoptical element used in the present invention.

FIG. 23 is a view for explanation of an ultra-high index lens.

FIGS. 24(a), 24(b), 24(c) and 24(d) are views for explanation of opticalpaths in an image display apparatus according to Example 14 of thepresent invention.

FIG. 25 is a view for examination of the size of an aperture in alight-blocking plate.

FIG. 26 shows the way in which the light-blocking plate is disposed inanother image display apparatus.

FIG. 27 shows the way in which the light-blocking plate is disposed instill another image display apparatus.

FIG. 28 shows the way in which the light-blocking plate is disposed in afurther image display apparatus.

FIG. 29 shows the way in which the light-blocking plate is disposed in astill further image display apparatus.

FIG. 30 shows the way in which the light-blocking plate is disposed in astill further image display apparatus.

FIG. 31 shows the way in which the light-blocking plate is disposed in astill further image display apparatus.

FIG. 32 shows the way in which the light-blocking plate is disposed in astill further image display apparatus.

FIG. 33 shows the way in which the light-blocking plate is disposed in astill further image display apparatus.

FIG. 34 shows the way in which the light-blocking plate is disposed in astill further image display apparatus.

FIG. 35 shows the way in which the light-blocking plate is disposed in astill further image display apparatus.

FIG. 36 shows the way in which the light-blocking plate is disposed in astill further image display apparatus.

FIG. 37 shows the way in which the light-blocking plate is disposed in astill further image display apparatus.

FIGS. 38(a), 38(b), 38(c) and 38(d) are views for explanation of opticalpaths in an image display apparatus according to a modification ofExample 14.

FIG. 39 is a view for explanation of an optical path in an image displayapparatus according to a further example.

FIG. 40 is a view for explanation of an optical path in an image displayapparatus according to a still further example.

FIG. 41 is a view for explanation of the structure and operation of alouver.

FIGS. 42(a), 42(b) and 42(c) are views for explanation of the operationof an image display apparatus according to a still further example.

FIGS. 43(a) and 43(b) are views for explanation of the operation of animage display apparatus according to a still further example.

FIGS. 44(a), 44(b), 44(c) and 44(d) are views for explanation of opticalpaths of display light and flare light in one image display apparatus.

FIG. 45 shows the whole arrangement of one example of a head-mountedimage display apparatus which uses an optical system according to thepresent invention.

FIG. 46 shows an arrangement in which an optical system according to thepresent invention is used as an imaging optical system.

FIG. 47 shows an arrangement of an optical system in which an opticalsystem according to the present invention is used as an imaging opticalsystem.

FIG. 48 shows an optical system of a conventional image displayapparatus.

FIG. 49 shows an optical system of another conventional image displayapparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, the reason for adopting the above-described arrangements,together with the operations thereof, will be explained, and then,examples of the present invention will be described.

First, the operation of an optical system according to the presentinvention as it is used as an image display apparatus will be explained.The operation of the optical system as it is used as an imaging opticalsystem is the same as that in the case of the image display apparatus.

The present invention relates to an layout of an optical system which isneeded in order to arrange the optical system in a compact form.

It is important to arrange the optical system in a thin form in order toreduce the overall thickness of the image display apparatus. As theimage display apparatus is reduced in thickness, the center of gravitycomes close to the center of the observer's head. Therefore, the momentof inertia can be reduced even if the weight is the same. In otherwords, the capability of the apparatus to follow up the movement of theobserver's head is markedly improved.

Accordingly, the present invention adopts an arrangement in which animage of an image display device is projected directly into anobserver's eyeball without using a relay optical system.

To form a thin optical system, the system is arrangement such that lightrays reciprocate in the optical system, and thus the optical path isfolded, thereby succeeding in reducing the thickness of the opticalsystem.

A wide observation field angle cannot be ensured simply by folding theoptical path of the optical system. Therefore, it is important to format least one reflecting surface by a curved surface having a concavesurface directed toward the pupil position of the optical system so thatlight rays are reflected and converged by the curved surface, and it isalso important to arrange the optical system such that light rays arerepeatedly reflected in the optical system.

The gist of the present invention resides in that a diffraction opticalelement (DOE) is used to correct even more satisfactorily aberrationsremaining in the optical system arranged as described above. Adiffraction optical element can refract light rays by diffraction actionof a diffraction surface formed on a thin substrate. This is veryimportant in producing a compact and lightweight optical system. Thatis, a diffraction optical element can be formed on a thin andlightweight substrate, whereas a refracting optical element of glass,i.e. an ordinary glass lens, is exceedingly heavy. Further, anaspherical surface action can also readily be imparted to a diffractionoptical element by varying the diffraction grating pitch on thediffraction surface in the radial direction, although a high level ofmanufacturing technique is usually required to realize an asphericalsurface action.

Further, it is important in order to correct coma and astigmatismproduced by a decentered optical member that the diffraction opticalelement should also be decentered in the same plane as the decentrationplane in which the optical member is decentered. The decentration of thediffraction optical element enables the coma and astigmatism to becorrected even more satisfactorily.

It is preferable that at least two of the at least three opticalsurfaces should be disposed with their concave surfaces directed towarda stop (eyeball). The reason for this is that field curvature producedby a concave mirror can be corrected by forming an optical system fromtwo reflecting surfaces, that is, one concave mirror having a concavesurface directed toward the stop and one convex mirror having a concavesurface directed toward the stop, with respect to light rays enteringthe optical system through the stop (light rays in backward ray tracingwhen the optical system is used in an image display apparatus). Itshould be noted that the decentered optical system does not fall underthe category of conventional optical systems which are rotationallysymmetric with respect to the optical axis; therefore, theabove-described arrangement must be considered in terms of twodirections, that is, the direction of the decentration plane in whichthe optical system is decentered, and the direction of a plane whichperpendicularly intersects both the stop and the decentration plane. Itis important to arrange the above-described positive and negativesurfaces, i.e. concave and convex surfaces, with respect to either ofthe two directions in which residual aberrations are greater, or inwhich greater importance is attached to the aberration correction.

Further, it is preferable that the at least three adjacent opticalsurfaces should be at least three reflecting surfaces having differentpowers. By doing so, the power can be effectively distributed, and itbecomes possible to project an image with smaller aberrations when thesame power is to be obtained. It should be noted that the term "opticalsurface" as used herein means a surface which intersects an opticalaxis.

By filling a space formed by the above-described three surfaces with amedium having a refractive index larger than 1, it becomes possible toform reflecting surfaces by using back-coated mirrors, and thus theoccurrence of comatic and spherical aberrations can be suppressed. Thisis because light rays in backward ray tracing from the pupil areconvergently refracted after passing through the second transmittingsurface, thereby making it possible to suppress divergence of light raysin the optical system more effectively than in the case of an opticalsystem using surface-coated mirrors when the same observation fieldangle is to be ensured. Thus, it becomes possible to reduce aberrationsproduced by the reflecting surfaces. At the same time, no vignetting oflight rays occurs, and it is possible to reduce the size of the opticalsystem.

Light rays emitted from the image display device pass through the firsttransmitting surface to enter a transparent medium which has at leastthree surfaces and whose refractive index is larger than 1. The incidentlight rays are reflected twice between the first and second reflectingsurfaces, whereby the optical path is folded. Then, the light rays passthrough the second transmitting surface, which lies between the stop andthe second reflecting surface, to exit from the transparent medium, andthen pass through the stop. With this arrangement, it is possible toform the first and second reflecting surfaces as back-coated mirrors.Consequently, it becomes possible to dispose concave and convex mirrorsat an appropriate interval such that aberrations, i.e. field curvatureand spherical aberration, produced by the reflecting surfaces of theconcave and convex mirrors cancel each other. Thus, it is possible tomaintain favorable aberration conditions.

It is desirable to arrange the optical system such that an imagedisplayed by the image display device is led to the observer's eyeballwithout forming an intermediate image on its way to the observer'seyeball. It is possible to provide a compact, lightweight and wide-fieldimage display apparatus by placing the image display device at the imageplane, disposing the observer's pupil at the stop position, andappropriately selecting various parameters of the optical system.

One of the at least three optical surfaces is formed such that thecurvature in the plane of decentration of the three surfaces isdifferent from the curvature in a plane perpendicularly intersecting thedecentration plane. By doing so, it becomes possible to correctastigmatism produced by the decentered concave mirror.

When field angles in two directions, i.e. vertical and horizontaldirections, are different from each other, aberration correction isdifferent for the direction of narrower field angle than for thedirection of wider field angle. Further, there is a difference in theway in which aberrations due to decentration occur between thedecentration direction and a direction in which the optical surfaces arenot decentered, and the target of aberration correction in the twoplanes differs according to each particular combination. Therefore, itis important to form an arbitrary optical surface such that thecurvatures in the two planes differ from each other. The arbitraryoptical surface is determined according to circumstances under which theoptical system is used.

Further, by making the curvature in the decentration plane differentfrom the curvature in a plane perpendicularly intersecting thedecentration plane, the aberration correcting load on the diffractionoptical element is reduced, so that satisfactory aberration correctioncan be made even if the diffraction optical element is produced as arotationally symmetric optical element.

If the first reflecting surface and the second transmitting surface aredisposed at the same position and with the same configuration, thenumber of surface configurations necessary to machine in the productionof the optical system reduces. Therefore, the production of the opticalsystem is facilitated. Even if the effective area of the firstreflecting surface and that of the second transmitting surface overlapeach other, light rays can be properly reflected and transmitted, and itbecomes possible to ensure a wide field angle.

Assuming that a light ray which emanates from the center of the displaysurface (display area) of the image display device and passes throughthe eyeball is defined as a principal ray, it is desirable for thesecond reflecting surface to be decentered with respect to the principalray. If the second reflecting surface is tilted with respect to theprincipal ray, it is possible to dispose the image surface (displaysurface) at a position sideward of the stop (eyeball pupil position).Accordingly, it becomes possible to reduce the amount by which theoptical system projects from the stop in the optical axis direction.

The second reflecting surface is preferably disposed with a concavesurface thereof directed toward the stop, and it is even more desirablefor the second reflecting surface to be formed by a back-coated mirror.By doing so, the second reflecting surface becomes a reflecting surfacehaving a relatively strong power in the optical system. Further, comaticaberration is minimized, and hence the coma correcting load imposed onanother surface is minimized. Thus, favorable results can be obtained.

Incidentally, diffraction optical elements have negative extremelystrong dispersion characteristics (Abbe's number: -3.45), as describedin Japanese Patent Application Laid-Open (KOKAI) No. 4-214516, filed bythe present applicant. Therefore, even a diffraction optical element ofweak power can exhibit powerful chromatic aberration correctioncapability. If a reflecting mirror is used to form a surface having aprincipal refracting power in the above-described optical system, lightrays can be refracted without producing chromatic aberration at all.However, chromatic aberration that is produced when light rays passthrough the first and second transmitting surfaces cannot be correctedby the reflecting surface and remains uncorrected. To correct thechromatic aberration, the diffraction optical element is designed toproduce chromatic aberration which corrects the chromatic aberrationproduced by the first and second transmitting surfaces, thereby enablingthe optical system to be satisfactorily corrected for chromaticaberration as a whole. Thus, it is possible to obtain a favorable image,which is corrected for chromatic aberration.

In a case where the first reflecting surface and the second transmittingsurface, which are disposed at the same position and with the sameconfiguration, are arranged to operate as a reflecting surface, thesurface is disposed such that light rays are incident thereon at anangle greater than the critical angle; in a case where the firstreflecting surface and the second transmitting surface are arranged tooperate as a transmitting surface, the surface is disposed such thatlight rays are incident thereon at an angle smaller than the criticalangle. By doing so, the loss of light quantity can be minimized, and itis possible to obtain an optical system that provides a bright image.

If the diffraction optical element is disposed between the opticalsystem and the stop (observer's eyeball pupil position), it is possibleto increase the focal length of the diffraction optical element.Accordingly, it becomes possible to effect aberration correction by adiffraction optical element which has a wide diffraction grating pitchand thus exhibits favorable productivity.

If the diffraction optical element is disposed between the opticalsystem and the image surface (display surface), the effective area ofthe diffraction optical element can be minimized, and it becomespossible to effect aberration correction by a small-sized diffractionoptical element.

It is desirable to satisfy the following condition:

    -0.1<1/f<0.1 (mm.sup.-1)                                   (1)

where f is the focal length of the diffraction optical element.

The condition (1) specifies the focal length of the diffraction opticalelement. If 1/f is not larger than the lower limit of the condition (1),i.e. -0.1, aberrations produced by the diffraction optical elementbecome excessively small in comparison to aberrations produced by therest of the optical system, resulting in under correction. Accordingly,it becomes impossible to effect favorable aberration correction. If 1/fis not smaller than the upper limit, i.e. 0.1, aberrations produced bythe diffraction optical element become excessively large, resulting inover correction with respect to aberrations produced by the rest of theoptical system.

In a case where the diffraction optical element is disposed between theoptical member and the observer's eyeball, it is desirable to satisfythe following condition:

    -0.1<1/f<0.1 (mm.sup.-1)                                   (2)

The condition (2) applies in a case where the diffraction opticalelement is disposed between the optical member and the stop. Thecondition (2) specifies the focal length of the diffraction opticalelement, as is the case with the condition (1). By satisfying thecondition (2), it becomes possible to effect favorable aberrationcorrection because aberrations produced by the diffraction opticalelement exactly cancel aberrations produced by the rest of the opticalsystem. The meaning of the upper and lower limits of the condition (2)is the same as in the condition (1).

Further, it is preferable from the viewpoint of aberration correction tosatisfy the following condition:

    -0.05<1/f<0.05 (mm.sup.-1)                                 (2)'

In a case where the diffraction optical element is disposed between theoptical member and the display surface, it is desirable to satisfy thefollowing condition:

    -0.05<1/f<0.05 (mm.sup.-1)                                 (3)

The condition (3) applies in a case where the diffraction opticalelement is disposed between the optical member and the image surface.The condition (3) specifies the focal length of the diffraction opticalelement, as is the case with the above-described conditions. Bysatisfying the condition (3), it becomes possible to effect favorableaberration correction because aberrations produced by the diffractionoptical element exactly cancel aberrations produced by the rest of theoptical system. The meaning of the upper and lower limits of thecondition (3) is the same as in the condition (1).

If a positioning device is provided such that the image display deviceis disposed at the image surface of the optical system and theobserver's eyeball is placed at the pupil position, it becomes possibleto construct a compact image display apparatus.

If the positioning device is one that enables the optical system to befitted on the observer's head, it becomes possible for the observer tosee the observation image in a desired posture and from a desireddirection. That is, the observer can see the observation image inhis/her own easy posture. Thus, it is possible to construct a compacthead-mounted image display apparatus which enables even a bedridden sickperson, for example, to see the observation image in a lying positionwith the image display apparatus fitted to his/her head.

If an image recording device is disposed in place of the image displaydevice of the optical system and a stop is provided at the eyeballposition in place of the observer's eye, it is possible to provide acompact imaging optical system. If the optical system is arranged toform an image of an object at an infinite distance, the optical systemcan be used as an imaging optical system, e.g. a camera finder opticalsystem, as shown in FIGS. 46 and 47.

Further, if the first reflecting surface and the second reflectingsurface are formed by a convex mirror and a concave mirror,respectively, which have their respective concave surfaces directedtoward the pupil, even more favorable results can be obtained in thecorrection of aberrations such as coma and field curvature.

Further, it is preferable to form the optical system from a singleoptical member and to combine it with a diffraction optical element. Bydoing so, it is possible to produce the whole optical system into asimple structure.

Assuming that a light ray which emanates from the center of an objectpoint and reaches the pupil center is defined as a principal ray, it ispreferable to satisfy the following condition:

    70°<θ.sub.1 <160°                      (4)

where θ₁ is an angle between the principal ray incident on the firstreflecting surface and the principal ray emanating from it.

The condition (4) determines the size of the optical system in thevertical direction. If θ₁ is not larger than the lower limit of thecondition (4), i.e. 70°, the first transmitting surface and secondreflecting surface of the optical system interfere with each other,making it impossible to obtain a wide observation field angle. If θ₁ isnot smaller than the upper limit, i.e. 160°, the optical systemlengthens in the vertical direction, making it difficult to achieve areduction in the size of the optical system.

It is even more desirable to satisfy the following condition:

    80°<θ.sub.1 <140°                      (4)

Assuming that a light ray which emanates from the center of an objectpoint and reaches the pupil center is defined as a principal ray, it isalso preferable to satisfy the following condition:

    30°<θ.sub.2 <120°                      (5)

where θ₂ is an angle between the principal ray incident on the secondreflecting surface and the principal ray emanating from it.

The condition (5) determines the size of the optical system in thevertical direction. If θ₂ is not larger than the lower limit of thecondition (5), i.e. 30°, the first transmitting surface and secondreflecting surface of the optical system interfere with each other,making it impossible to obtain a wide observation field angle. If θ₂ isnot smaller than the upper limit, i.e. 120°, the optical systemlengthens in the vertical direction, making it difficult to achieve areduction in the size of the optical system.

It is even more desirable to satisfy the following condition:

    35°<θ.sub.2 <70°                       (5)'

In a case where the diffraction optical element is produced as arotationally symmetric optical element, it is preferable to decenter thediffraction optical element with respect to the principal ray. Regardingthe amount of eccentricity of the diffraction optical element in thiscase, it is preferable to satisfy the following conditions:

    -50<d<50 (mm)                                              (6)

    -50 <α<50 (°)                                 (7)

where d is the amount of decentering, and α is the amount of tilt.

If d or α is not within the range defined by the above condition (6) or(7), the amount of eccentricity of the diffraction optical elementbecomes undesirably large, making it impossible for the diffractionoptical element to correct, with good balance, coma and astigmatismproduced in the optical system on account of decentration. Accordingly,favorable aberration correction cannot be attained.

It is even more desirable to satisfy the following conditions:

    -10<d<10 (mm)                                              (6)'

    -10<α<20 (°)                                  (7)'

By satisfying the above conditions (6)' and (7)', it is possible toeffect even more favorable aberration correction.

Let us take notice of a principal ray which passes through the center ofthe stop and reaches the center of the image surface, and a light raywhich passes through the stop at a height h in the vicinity of theprincipal ray. The focal length of the entire optical system in thevicinity of the principal ray can be obtained by

    f=h/sin(tan.sup.-1 u.sub.1 -tan.sup.-1 u.sub.2)

where u₁ is an angle of intersection of the principal ray and the imagesurface, and u₂ is an angle of intersection of the light ray passingthrough the stop at the height h in the vicinity of the principal rayand the image surface.

It is preferable to satisfy the following condition:

    -2<F<2                                                     (8)

where F is the value of (the focal length f of the entire opticalsystem)/(the focal length f of the diffraction optical element).

The condition (8) also determines a balance between the amount ofaberration produced in the entire optical system and the amount ofaberration correction made by the diffraction optical element. If F isnot larger than the lower limit, i.e. -2, or not smaller than the upperlimit, i.e. 2, the amount of aberration correction made by thediffraction optical element becomes undesirably large, making itimpossible for the diffraction optical element to correct, with goodbalance, field curvature and chromatic aberration produced in theoptical system. Accordingly, it is impossible to attain favorableaberration correction.

It is even more desirable to satisfy the following condition:

    -1<F<1                                                     (8)'

Next, the reason for adopting the above-described arrangements in theimage display apparatus according to the present invention, togetherwith the operations thereof, will be explained.

In the optical system of the image display apparatus according to thepresent invention, a diffraction optical element (hereinafter referredto as "DOE"), represented by a Fresnel zone plate, or a gradient indexlens is used to correct chromatic aberration, field curvature and otheraberrations remaining uncorrected in a single decentered prism which hasthree or four optical surfaces and in which a space formed between theseoptical surfaces is filled with a medium having a refractive indexlarger than 1. The aberration correction capabilities of DOEs andgradient index lenses will be explained below.

DOEs, represented by zone plates, have high reciprocal dispersioncharacteristics i.e. Abbe's number ν_(d) =-3.45, and exhibit powerfulchromatic aberration correction capability. Accordingly, a DOE caneffectively correct chromatic aberration remaining in a singledecentered prism as a result of the achievement of high-density imagedisplay devices, as described above.

Further, because a DOE having aspherical action can be produced by thesame method as that for a DOE having spherical action, it is possible topositively give aspherical action to the DOE and hence possible toeffectively correct off-axis aberration increased as a result ofachievement of a wider field of view. In this case, if the DOE is givensuch aspherical action (pitch distribution) that the power becomesweaker than the power of a paraxial spherical system as the distancefrom the optical axis increases, the aberration correction capabilityincreases. Further, with such pitch arrangement, the pitch at theperiphery of the clear aperture region of the DOE becomes relativelylarge, so that the productivity of the DOE also improves. In addition,unlike a refracting lens, a DOE can be produced simply by forming adiffraction surface on the surface of a substrate. Therefore, it isaccompanied by practically no increase in volume or weight and hencefavorable for use in an optical system of a head-mounted image displayapparatus.

Noting that a gradient index lens is capable of correcting Petzval sumand chromatic aberration, the present invention uses a gradient indexlens in combination with the above-described single decentered prism tocorrect chromatic aberration, field curvature and other aberrationsremaining in the decentered prism. The Petzval sum and chromaticaberration correcting action of a gradient index lens will be explainedbelow.

A gradient index lens used in the present invention is of the radialtype that has a refractive index distribution in a directionperpendicular to the optical axis. The refractive index distribution forthe reference wavelength is expressed by

    n(r)=N.sub.0 +N.sub.1 r.sup.2 +N.sub.2 r.sup.4 +N.sub.3 r.sup.6 +(9)

where N₀ is the refractive index for the reference wavelength of thelens on the optical axis, r is the radial distance from the opticalaxis, n(r) is the refractive index for the reference wavelength at aposition of distance r from the optical axis, and N₁, N₂, N₃, . . . are2nd-, 4th- and 6th-order coefficients of the reference wavelength,respectively.

First of all, the correction of Petzval sum will be explained. Amongquantities that must be particularly noted at the stage of initiallydesigning a lens system is Petzval sum, which is determined by powerdistribution. The Petzval sum of a homogeneous system may be expressedby

    .o slashed..sub.s /N.sub.0                                 (10)

where .o slashed._(s) is the refracting power of the surface, and N₀ isthe refractive index of the lens on the optical axis.

The Petzval sum of a single gradient index lens may be expressed by

    .o slashed..sub.s '/N.sub.0 +.o slashed..sub.M /N.sub.0.sup.2(11)

where .o slashed._(s) ' is the refracting power of the surface, and .oslashed._(M) is the refracting power of the medium.

As will be clear from the expression (11), a gradient index lens cancorrect Petzval sum because its medium has refracting power.

Next, the correction of chromatic aberration will be explained. In thecase of a gradient index lens, a medium thereof also has capability ofcorrecting chromatic aberration. The condition that must be satisfied bya single gradient index lens to correct axial chromatic aberration is asfollows:

    .o slashed..sub.s '/ν.sub.0d +.o slashed..sub.M /ν.sub.1d =0(12)

Assuming the refractive indices of the lens on the optical axis for thed-line, F-line and C-line are N_(0d), N_(0F) and N_(0C), respectively,ν_(0d) is expressed by

    ν.sub.0d =(N.sub.0d -1)/(N.sub.0F -N.sub.0C)            (13)

From 2nd-order coefficients N_(1d), N_(1F) and N_(1C) in the refractiveindex distribution expression (9) for the d-line, F-line and C-line,ν_(1d) is obtained as follows:

    ν.sub.1d =N.sub.1d /(N.sub.1F -N.sub.1C)                (14)

In other words, it is possible to correct chromatic aberration byvarying the medium distribution configuration of a gradient index lensfor each wavelength.

In the present invention, chromatic aberration, field curvature andother aberrations which remain in a decentered prism having three orfour optical surfaces, as shown in FIGS. 14 to 20, are corrected byproducing aberrations which are opposite in sign to the residualaberrations with a correction optical element comprising theabove-described diffraction optical element or gradient index lens. Sucha correction optical element may be disposed on either the observer'seyeball side or the image display device side of the decentered prism(optical member).

In a case where a diffraction optical element is used as a correctionoptical element, it is desirable to satisfy the following condition:

    -1<1/f<1                                                   (a)

where f (mm) is the focal length of the diffraction optical element.

It is more desirable to satisfy the following condition:

    -0.1<1/f<0.1                                               (a')

It is even more desirable to satisfy the following condition:

    0<1/f<0.01                                                 (a")

In a case where a gradient index lens is used as a correction opticalelement, it is desirable to satisfy the following condition:

    0.5<N0/N1<1.5                                              (b)

where N0 is the refractive index at the center of the gradient indexlens, and N1 is the refractive index at the periphery of the gradientindex lens.

It is more desirable to satisfy the following condition:

    0.8<N0/N1<1.2                                              (b')

The conditions (a), (a'), (a"), (b) and (b') are conditions that must besatisfied for the diffraction optical element or gradient index lens tohave approximately no power because it is desirable to use anapproximately non-power diffraction optical element or gradient indexlens in order to effect the desired aberration correction withoutsubstantially changing the eye point, focal length, magnification, etc.of the entire ocular optical system.

Next, some numerical examples in a case where the optical systemaccording to the present invention is arranged in the form of an imagedisplay apparatus will be explained with reference to the accompanyingdrawings.

First, a method of designing an optical system including a DOE used inthe present invention will be explained.

The principle of a DOE, which is an optical element based on adiffractive phenomenon, is detailed, for example, in Chapters VI and VIIof "Small-Sized Optical Elements for Optical Designers" (Optronics). Letus explain it briefly.

In the case of an optical element based on a refractive phenomenon, alight ray 1 is bent, as shown in FIG. 21, on the basis of Snell's lawgiven by

    n·sinθ=n'·sinθ'              (15)

where

n: the refractive index of the entrance-side medium

n': the refractive index of the exit-side medium

θ: the incident angle of the ray

θ': the exit angle of the ray

On the other hand, in the case of a DOE, a light ray 1 is bent, as shownin FIG. 22, by a diffractive phenomenon expressed by

    n·sinθ-n'·sinθ'=mλ/d  (16)

where

n: the refractive index of the entrance-side medium

n': the refractive index of the exit-side medium

θ: the incident angle of the ray

θ': the exit angle of the ray

m: the order of diffraction

λ: the wavelength

d: the pitch of the DOE

It should be noted that if the DOE is blazed or approximatively blazed,high diffraction efficiency can be maintained.

As a technique of designing an optical system including a DOE, Sweattmodel is known; this is detailed in W. C. Sweatt "NEW METHODS OFDESIGNING HOLOGRAPHIC OPTICAL ELEMENTS", SPIE, Vol. 126, pp. 46-53(1977). Sweatt model will be briefly explained below with reference toFIG. 23.

In FIG. 23, reference numeral 5 denotes a refracting lens (ultra-highindex lens) in which n>>1, and 2 normal line. Reference symbol z denotescoordinates in the direction of an optical axis, h coordinates in thedirection lying along the substrate.

According to the above-mentioned paper, the following equation holds:

    (n.sub.u -1)dz/dh=n·sinθ-n'·sinθ'(17)

where

n_(u) : the refractive index of the ultra-high index lens (n_(u) =1001in the design explained below)

z: the coordinates in the optical axis direction of the ultra-high indexlens

h: the coordinates along the medium of the ultra-high index lens

n: the refractive index of the entrance-side medium

n': the refractive index of the exit-side medium

θ: the incident angle of the ray

θ': the exit angle of the ray

Therefore, the following equation holds from Eqs. (16) and (17):

    (n.sub.u -1)dz/dh=mλ/d                              (18)

That is, the equivalent relationship expressed by Eq. (18) isestablished between "the surface configuration of the refracting lens inwhich n>>1" and "the pitch of the DOE". Accordingly, the pitchdistribution on the DOE can be obtained from the surface configurationof the ultra-high index lens designed on the basis of Sweatt model.

More specifically, let us assume that the ultra-high index lens isdesigned as an aspherical lens defined by

    z=ch.sup.2 /{1+ 1-c.sup.2 (k+1)h.sup.2 !.sup.1/2 }+Ah.sup.4 +Bh.sup.6 +Ch.sup.8 +Dh.sup.10                                      (19)

where

z: the displacement (sag value) from a plane tangent to the lens at theoptical axis

c: the curvature

h: the distance from the optical axis

k: the conical constant

A: the 4th-order aspherical coefficient

B: the 6th-order aspherical coefficient

C: the 8th-order aspherical coefficient

D: the 10th-order aspherical coefficient

Assuming that one surface of the ultra-high index lens is a planesurface for simplification of the explanation, the following equation isobtained from Eqs. (18) and (19), ##EQU1##

Thus, the DOE should be given a pitch distribution defined by Eq. (20).

Further, it is necessary for Eq. (18) to hold for any desiredwavelength.

    ∴n(λ)-1=Kλ                           (21)

where K=m/ d·dz/dh!

Since n_(d) is herein assumed to be 1001, K=1.7020.

Thus, the dispersion characteristics of the DOE can be expressedaccording to Eq. (21) by assuming that n_(C) =1118.0, N_(e) =930.39,n_(F) =828.37 and n_(g) =742.78.

Although in the following examples aspherical surface terms for only4th- to 10th-orders are used, it should be noted that aspherical surfaceterms for 12th-, 14th- . . . orders may be used, as a matter of course.

In those of the following examples which use a DOE, only one DOE isused. However, two or more DOEs may be used, as a matter of course.

Next, the optical system according to the present invention will bedescribed with reference to FIGS. 1 to 13 which are sectional views ofoptical systems for a single eye according to Examples 1 to 13 in whichthe optical system is used in an image display apparatus.

It should be noted that the following explanation will be made on thebasis of backward ray tracing in which light rays are traced from thepupil position toward the image display device. The object position is avirtual image position which is -1 m away from the pupil.

Constituent parameters of each example will be shown later. In thefollowing description, the surface Nos. are shown as ordinal numbers inbackward tracing from an exit pupil position (observer's pupil position)1 of an optical system toward an image display device 6. A coordinatesystem is defined as shown in FIGS. 1 to 8: With the center of theoptical system exit pupil 1 defined as an origin, the direction of anobserver's visual axis 2 is taken as a Z-axis. Regarding the sign of theZ-axis, the direction that extends away from the center of the pupil 1is defined as a positive direction. A direction which is perpendicularto the Z-axis in the plane of the figure is taken as a Y-axis, where theupward direction as viewed in the figure is defined as a positivedirection. A direction perpendicularly intersecting both the Z- andY-axes, i.e. a direction perpendicular to the plane of the figure, istaken as an X-axis. It should be noted that the direction of the X-axisthat extends from the obverse side to the reverse side of the plane ofthe figure is defined as a positive direction.

In constituent parameters of Examples 1 to 7 (shown later), regardingeach surface for which eccentricities Y and Z and tilt angle θ areshown, the eccentricity Y is a distance by which the vertex of thesurface decenters in the Y-axis direction from the surface No. 1 (pupilposition 1) as a reference surface, and the eccentricity Z is a distanceby which the vertex of the surface decenters in the Z-axis directionfrom the reference surface. The tilt angle θ is the tilt angle of thecentral axis of the surface from the Z-axis. In this case, positive θmeans counterclockwise rotation. The three surfaces of the diffractionoptical element, i.e. from the substrate surface to the diffractionsurface, are coaxial with respect to each other; therefore,eccentricities Y and Z and tilt angle θ are shown only for thediffraction optical element substrate surface. In this case, theeccentricity Y is a distance by which the vertex of the substratesurface decenters in the Y-axis direction from the surface No. 1 (pupilposition 1), and the eccentricity Z is a distance by which the vertex ofthe substrate surface decenters in the Z-axis from the surface No. 1.The tilt angle θ is the tilt angle of the central axis of the substratesurface from the Z-axis. Regarding the other surfaces, the positionalarrangement of each surface is shown by the surface separation.

In constituent parameters of Examples 8 to 13 (shown later), regardingeach surface for which eccentricities Y and Z and tilt angle θ areshown, the eccentricity Y is a distance by which the vertex of thesurface decenters in the Y-axis direction from the surface No. 1 (pupilposition 1) as a reference surface, and the eccentricity Z is a distanceby which the vertex of the surface decenters in the Z-axis directionfrom the reference surface. The tilt angle θ is the tilt angle of thecentral axis of the surface from the Z-axis. In this case, positive θmeans counterclockwise rotation. It should be noted that a surfacewithout indication of eccentricities Y, Z and tilt angle θ is coaxialwith respect to the preceding surface. Surface separations are shownonly for coaxial portions. The surface separation is the axial distancefrom the surface concerned to the next surface. It should be noted thatsurface separations are shown with the direction of backward tracingalong the optical axis defined as positive direction.

The non-rotationally symmetric aspherical configuration of each surfacemay be expressed in the coordinate system defining the surface asfollows: ##EQU2##

where R_(y) is the paraxial curvature radius of the surface in theYZ-plane (the plane of the figure); R_(x) is the paraxial curvatureradius in the XZ-plane; K_(x) is the conical coefficient in theXZ-plane; K_(y) is the conical coefficient in the YZ-plane; AR, BR, CRand DR are 4th-, 6th-, 8th- and 10th-order aspherical coefficients,respectively, which are rotationally symmetric with respect to theZ-axis; and AP, BP, CP and DP are 4th-, 6th-, 8th- and 10th-orderaspherical coefficients, respectively, which are rotationally asymmetricwith respect to the Z-axis.

The rotationally symmetric aspherical configuration of each surface maybe expressed in the coordinate system defining the surface as follows:

    Z= (h.sup.2 /R)/ 1+{1-(1+K)(h.sup.2 /R.sup.2)}.sup.1/2 !+Ah.sup.4 +Bh.sup.6 +Ch.sup.8 +Dh.sup.10 (h.sup.2 =X.sup.2 +Y.sup.2)

where R is the paraxial curvature radius; K is the conical coefficient;and A, B, C and D are 4th-, 6th-, 8th- and 10th-order asphericalcoefficients, respectively.

In the coordinate system of each of the formulae that express surfaceconfigurations, the vertex of each surface is defined as an origin, andthe center axis of each surface is defined as a Z-axis.

In constituent parameters (shown later), those which are not given anyvalues are zero. The refractive index of a medium lying between a pairof surfaces is expressed by the refractive index for the spectral d-line(wavelength: 587.56 nm). Lengths are given in millimeters.

In an actual apparatus, needless to say, the direction in which lightrays are reflected by the optical system may be either of the upward andsideward directions of the observer.

The optical system according to the present invention is also usable asan imaging optical system that forms an image of a distant object point.

FIGS. 1 to 7 are sectional views of image display apparatuses designedfor a single eye according to Examples 1 to 7.

In each sectional view, reference numeral 1 denotes an observer's pupilposition (exit pupil position), 2 an observer's visual axis, 6 an imagedisplay device, 7 an optical member, 8 a diffraction optical element, 3a first surface of the optical member 7, 4 a second surface of theoptical member 7, and 5 a third surface of the optical member 7.

The actual path of light rays in each example is as follows: In Examples1, 2, 4 and 7, a bundle of light rays emitted from the image displaydevice 6 enters the optical member 7 through the diffraction opticalelement 8 while being refracted by the third surface 5 of the opticalmember 7. The incident ray bundle is internally reflected by the firstsurface 3 and then reflected by the second surface 4 so as to beincident on the first surface 3 again. The ray bundle is refracted bythe first surface 3 and projected into the observer's eyeball with theobserver's iris position or eyeball rolling center as the exit pupil 1.In these examples, the first transmitting surface is the third surface5, the first reflecting surface is the first surface 3, the secondreflecting surface is the second surface 4, and the second transmittingsurface is the first surface 3. The first reflecting surface and thesecond transmitting surface are provided at the same position and withthe same configuration.

In Examples 3 and 6, a bundle of light rays emitted from the imagedisplay device 6 enters the optical member 7 while being refracted bythe third surface 5 of the optical member 7. The incident ray bundle isinternally reflected by the first surface 3 and then reflected by thesecond surface 4 so as to be incident on the first surface 3 again. Theray bundle exits from the optical member 7 while being refracted by thefirst surface 3, and is projected through the diffraction opticalelement 8 into the observer's eyeball with the observer's iris positionor eyeball rolling center as the exit pupil 1. In these examples, thefirst transmitting surface is the third surface 5, the first reflectingsurface is the first surface 3, the second reflecting surface is thesecond surface 4, and the second transmitting surface is the firstsurface 3. The first reflecting surface and the second transmittingsurface are provided at the same position and with the sameconfiguration.

In Example 5, a bundle of light rays emitted from the image displaydevice 6 enters the optical member 7 through the diffraction opticalelement 8 while being refracted by the third surface 5 of the opticalmember 7. The incident ray bundle is internally reflected by the firstsurface 3 so as to be incident on the third surface 5. This time, theray bundle is internally reflected by the third surface 5. The reflectedray bundle is internally reflected by the first surface 3 and thenreflected by the second surface 4 so as to be incident on the firstsurface 3 once again. This time, the ray bundle is refracted by thefirst surface 3 and projected into the observer's eyeball with theobserver's iris position or eyeball rolling center as the exit pupil 1.In this example, the first transmitting surface is the third surface 5,the first reflecting surface is the first surface 3, the secondreflecting surface is the third surface 5, the third reflecting surfaceis the first surface 3, the fourth reflecting surface is the secondsurface 4, and the second transmitting surface is the first surface 3.The first transmitting surface and the second reflecting surface areprovided at the same position and with the same configuration. The firstreflecting surface, the third reflecting surface and the secondtransmitting surface are provided at the same position and with the sameconfiguration.

Field angles and pupil diameters in Examples 1 to 7 are as follows:

Example 1:

The horizontal field angle is 40°, the vertical field angle is 30°, andthe pupil diameter is 4 millimeters.

Example 2:

The horizontal field angle is 40°, the vertical field angle is 30°, andthe pupil diameter is 4 millimeters.

Example 3:

The horizontal field angle is 40°, the vertical field angle is 30°, andthe pupil diameter is 4 millimeters.

Example 4:

The horizontal field angle is 40°, the vertical field angle is 30°, andthe pupil diameter is 4 millimeters.

Example 5:

The horizontal field angle is 30°, the vertical field angle is 22.5°,and the pupil diameter is 4 millimeters.

Example 6:

The horizontal field angle is 40°, the vertical field angle is 30°, andthe pupil diameter is 4 millimeters.

Example 7:

The horizontal field angle is 40°, the vertical field angle is 30°, andthe pupil diameter is 4 millimeters.

FIGS. 8 to 13 are sectional views of image display apparatuses designedfor a single eye according to Examples 8 to 13. In the figures,reference numeral 1 denotes an observer's pupil position, 2 anobserver's visual axis, 6 an image display device, 10 an ocular opticalsystem, 5 a first surface of the ocular optical system 10, 4 a secondsurface of the ocular optical system 10, 3 a third surface of the ocularoptical system 10, 9 a fourth surface of the ocular optical system 10, 7a decentered prism, 8 a DOE, and 13 a gradient index lens (hereinafterreferred to as "GRIN").

The actual path of light rays in each of Examples 8 to 13 is as follows:A bundle of light rays emitted from the image display device 6 entersthe ocular optical system 10 while being refracted by the first surface5 of the ocular optical system 10. The incident ray bundle is internallyreflected by the fourth surface (the third surface 3 also serves as thefourth surface) 9 and then reflected by the second surface 4 so as to beincident on the third surface 3 again. The ray bundle is refracted bythe third surface 3 and projected into the observer's eyeball with theobserver iris position or eyeball rolling center as the exit pupil 1.

Example 8:

In this example, as shown in the sectional view of FIG. 8, thehorizontal field angle is 45.4°, the vertical field angle is 34.4°, andthe pupil diameter is 4 millimeters. In this example, an approximatelynon-power DOE 8 is disposed between the exit pupil 1 and the decenteredprism 7. In the constituent parameters (shown later), the surface Nos.5, 6 and 7 are anamorphic aspherical surfaces, and the surface No. 8 isa plane surface.

Example 9:

In this example, as shown in the sectional view of FIG. 9, thehorizontal field angle is 45.4°, the vertical field angle is 34.4°, andthe pupil diameter is 4 millimeters. In this example, an approximatelynon-power DOE 8 is disposed between the decentered prism 7 and the imagedisplay device 6. In the constituent parameters (shown later), thesurface Nos. 2, 3 and 4 are anamorphic aspherical surfaces, and thesurface No. 5 is a plane surface.

Example 10:

In this example, as shown in the sectional view of FIG. 10, thehorizontal field angle is 45.4°, the vertical field angle is 34.4°, andthe pupil diameter is 4 millimeters. In this example, an approximatelynon-power GRIN 13 having plane surfaces on both sides thereof isdisposed between the decentered prism 7 and the image display device 6.In the constituent parameters (shown later), the surface Nos. 2, 3 and 4are anamorphic aspherical surfaces, and the surface No. 5 is a planesurface.

Example 11:

In this example, as shown in the sectional view of FIG. 11, thehorizontal field angle is 45.4°, the vertical field angle is 34.4°, andthe pupil diameter is 4 millimeters. In this example, an approximatelynon-power GRIN 13 having plane surfaces on both sides thereof isdisposed between the exit pupil 1 and the decentered prism 7. In theconstituent parameters (shown later), the surface Nos. 4, 5 and 6 areanamorphic aspherical surfaces, and the surface No. 7 is a planesurface.

Example 12:

In this example, as shown in the sectional view of FIG. 12, thehorizontal field angle is 45.4°, the vertical field angle is 34.4°, andthe pupil diameter is 4 millimeters. In this example, an approximatelynon-power meniscus-shaped GRIN 13 having a concave surface directedtoward the image display device 6 is disposed between the decenteredprism 7 and the image display device 6. In the constituent parameters(shown later), the surface Nos. 2, 3 and 4 are anamorphic asphericalsurfaces, and the surface No. 5 is a plane surface.

Example 13:

In this example, as shown in the sectional view of FIG. 13, thehorizontal field angle is 45.4°, the vertical field angle is 34.4°, andthe pupil diameter is 4 millimeters. In this example, an approximatelynon-power meniscus-shaped GRIN 13 having a convex surface directedtoward the image display device 6 is disposed between the decenteredprism 7 and the image display device 6. In the constituent parameters(shown later), the surface Nos. 2, 3 and 4 are anamorphic asphericalsurfaces, and the surface No. 5 is a plane surface.

Constituent parameters in the above-described Examples 1 to 13 will beshown below. It should be noted that the above-described N_(1d), N_(1F)and N_(1C) are also shown in regard to the GRIN 13.

    __________________________________________________________________________                          Refractive                                              Surface                                                                             Radius of  Surface                                                                            index  Abbe's No.                                       No.   curvature  separation                                                                         (Eccentricity)                                                                       (Tilt angle)                                     __________________________________________________________________________    Example 1                                                                     1     ∞(pupil)                                                          2     R.sub.y -71.17939                                                                             1.4922 57.50                                                  R.sub.x -64.5987                                                                              Y 43.462                                                                             θ 21.63°                                  K.sub.y 0       Z 24.298                                                      K.sub.x 0                                                                     AR 0.189094 × 10.sup.-5                                                 BR 0.502157 × 10.sup.-15                                                CR -0.279487 × 10.sup.-13                                               DR -0.109879 × 10.sup.-15                                               AP -0.164076                                                                  BP 53.350587                                                                  CP 0.031156                                                                   DP -0.572366                                                            3     R.sub.y -77.512310                                                                            1.4922 57.50                                                  R.sub.x -59.71490                                                                             Y 14.711                                                                             θ -10.29°                                 K.sub.y 0       Z 46.484                                                      K.sub.x 0                                                                     AR -0.869330 × 10.sup.-12                                               BR 0.233397 × 10.sup.-10                                                CR -0.939132 × 10.sup.-13                                               DR -0.269891 × 10.sup.-15                                               AP -341.628501                                                                BP 1.998719                                                                   CP 0.538375                                                                   DP -0.017896                                                            4     R.sub.y -71.17939                                                                             1.4922 57.50                                                  R.sub.x -64.5987                                                                              Y 43.462                                                                             θ 21.63°                                  K.sub.y 0       Z 24.298                                                      K.sub.x 0                                                                     AR 0.189094 × 10.sup.-5                                                 BR 0.502157 × 10.sup.-15                                                CR -0.279487 × 10.sup.-13                                               DR -0.109879 × 10.sup.-15                                               AP -0.164076                                                                  BP 53.350587                                                                  CP 0.031156                                                                   DP -0.572366                                                            5     R.sub.y -52.225998                                                                            Y 29.105                                                                             θ 78.19°                                  R.sub.x -39.65538                                                                             Z 21.624                                                      K.sub.y 0                                                                     K.sub.x 0                                                                     AR 0.290484 × 10.sup.-4                                                 BR -0.875623 × 10.sup.-7                                                CR -0.144496 × 10.sup.-9                                                DR 0.779302 × 10.sup.-13                                                AP -0.787876                                                                  BP -0.424320                                                                  CP -0.261606                                                                  DP -0.191974                                                            6     ∞         1.5163 64.10                                                                  Y 23.095                                                                             θ 49.79°                                                  Z 39.274                                                7     ∞ (DOE)                                                                            0.000                                                                              1001   -3.45                                            8     -244948.903275                                                                K 0                                                                           A -0.590641 × 10.sup.-8                                                 B 0.524567 × 10.sup.-10                                                 C 0.914818 × 10.sup.-13                                                 D -0.907250 × 10.sup.-15                                          9     ∞(display surface)                                                                      Y 27.171                                                                             θ 39.16°                                                  Z 43.254                                                Example 2                                                                     1     ∞(pupil)                                                          2     R.sub.y -79.38031                                                                             1.4922 57.50                                                  R.sub.x -80.18493                                                                             Y 41.448                                                                             θ 21.53°                                  K.sub.y 0       Z 23.485                                                      K.sub.x 0                                                                     AR 0.146956 × 10.sup.-5                                                 BR 0.549579 × 10.sup.-15                                                CR -0.258466 × 10.sup.-13                                               DR -0.732260 × 10.sup.-15                                               AP -0.147439                                                                  BP 55.826649                                                                  CP 0.012500                                                                   DP -0.775575                                                            3     R.sub.y -80.70890                                                                             1.4922 57.50                                                  R.sub.x -61.92076                                                                             Y 10.566                                                                             θ -12.08°                                 K.sub.y 0       Z 45.313                                                      K.sub.x 0                                                                     AR -0.37995 × 10.sup.-11                                                BR 0.587113 × 10.sup.-10                                                CR -0.577526 × 10.sup.-13                                               DR -0.107906 × 10.sup.-14                                               AP -151.584065                                                                BP 1.826841                                                                   CP 0.952204                                                                   DP -0.242091                                                            4     R.sub.y -79.38031                                                                             1.4922 57.50                                                  R.sub.x -80.18493                                                                             Y 41.448                                                                             θ 21.53°                                  K.sub.y 0       Z 23.485                                                      K.sub.x 0                                                                     AR 0.146956 × 10.sup.-5                                                 BR 0.549579 × 10.sup.-15                                                CR -0.258466 × 10.sup.-13                                               DR -0.732260 × 10.sup.-15                                               AP -0.147439                                                                  BP 55.826649                                                                  CP 0.012500                                                                   DP -0.775575                                                            5     R.sub.y -42.27641                                                                             Y 27.378                                                                             θ 84.34°                                  R.sub.x -89.13397                                                                             Z 21.361                                                      K.sub.y 0                                                                     K.sub.x 0                                                                     AR 0.362749 × 10.sup.-7                                                 BR -0.123433 × 10.sup.-7                                                CR 0                                                                          DR 0                                                                          AP -14.544698                                                                 BP 0.088670                                                                   CP 0                                                                          DP 0                                                                    6     ∞    0.250                                                                              1.5163 64.10                                                                  Y 27.521                                                                             θ 44.04°                                                  Z 35.248                                                7     ∞ (DOE)                                                                            0.000                                                                              1001   -3.45                                            8     -259967.754835                                                                K 0                                                                           A 0.295513 × 10.sup.-7                                                  B -0.270162 × 10.sup.-10                                                C 0.223860 × 10.sup.-12                                                 D -0.328751 × 10.sup.-15                                          9     ∞(display surface)                                                                      Y 26.947                                                                             θ 38.98°                                                  Z 42.638                                                Example 3                                                                     1     ∞(pupil)                                                          2     ∞    0.280                                                                              1.5163 64.10                                                                  Y 4.361                                                                              θ 6.46°                                                   Z 30.506                                                3     ∞ (DOE)                                                                            0.000                                                                              1001   -3.45                                            4     -1210471.75072                                                                K 0                                                                           A 0.681966 × 10.sup.9                                                   B -0.294141 × 10.sup.11                                                 C 0.635773 × 10.sup.-14                                                 D -0.316842 × 10.sup.-17                                          5     R.sub.y -91.85932                                                                             1.4922 57.50                                                  R.sub.x -85.77710                                                                             Y 42.786                                                                             θ 25.01°                                  K.sub.y 0       Z 21.186                                                      K.sub.x 0                                                                     AR 0.150944 × 10.sup.-5                                                 BR 0.581341 × 10.sup.-15                                                CR -0.309854 × 10.sup.-13                                               DR -0.622017 × 10.sup.-15                                               AP -0.238195                                                                  BP 55.180495                                                                  CP 0.096230                                                                   DP -0.784416                                                            6     R.sub.y -80.263766                                                                            1.4922 57.50                                                  R.sub.x -65.58356                                                                             Y 12.859                                                                             θ -8.05°                                  K.sub.y 0       Z 46.986                                                      K.sub.x 0                                                                     AR -0.320509 × 10.sup.-8                                                BR 0.441212 × 10.sup.-10                                                CR -0.320183 × 10.sup.-12                                               DR -0.790570 × 10.sup.-15                                               AP -0.748250                                                                  BP 1.67572                                                                    CP 0.472263                                                                   DP -0.340182                                                            7     R.sub.y -91.85932                                                                             1.4922 57.50                                                  R.sub.x -85.77710                                                                             Y 42.786                                                                             θ 25.01°                                  K.sub.y 0       Z 21.186                                                      K.sub.x 0                                                                     AR 0.150944 × 10.sup.-5                                                 BR 0.581341 × 10.sup.-15                                                CR -0.309854 × 10.sup.-13                                               DR -0.622017 × 10.sup.-15                                               AP -0.238195                                                                  BP 55.180495                                                                  CP 0.096230                                                                   DP -0.784416                                                            8     R.sub.y -60.476554                                                                            Y 28.128                                                                             θ 81.46°                                  R.sub.x -34.92352                                                                             Z 20.175                                                      K.sub.y 0                                                                     K.sub.x 0                                                                     AR 0.445485 × 10.sup.-4                                                 BR -0.169231 × 10.sup.-6                                                CR 0.331085 × 10.sup.-9                                                 DR -0.217700 × 10.sup.-12                                               AP -0.785547                                                                  BP -0.518386                                                                  CP -0.402954                                                                  DP -0.355235                                                            9     ∞(display surface)                                                                      Y 27.946                                                                             θ 47.35°                                                  Z 40.648                                                Example 4                                                                     1     ∞(pupil)                                                          2     R.sub.y -63.687638                                                                            1.4922 57.50                                                  R.sub.x -52.93087                                                                             Y 42.650                                                                             θ 21.79°                                  K.sub.y 0       Z 25.411                                                      K.sub.x 0                                                                     AR 0.237756 × 10.sup.-5                                                 BR 0.497821 × 10.sup.-15                                                CR -0.279870 × 10.sup.-13                                               DR -0.880162 × 10.sup.-16                                               AP -0.194847                                                                  BP 54.229862                                                                  CP -0.082177                                                                  DP -0.563852                                                            3     R.sub.y -74.468047                                                                            1.4922 57.50                                                  R.sub.x -56.85709                                                                             Y 12.514                                                                             θ -12.67°                                 K.sub.y 0       Z 46.582                                                      K.sub.x 0                                                                     AR -0.175162 × 10.sup.-12                                               BR 0.101402 × 10.sup.-10                                                CR -0.133271 × 10.sup.-12                                               DR -0.313264 × 10.sup.-15                                               AP -102.638843                                                                BP 2.769603                                                                   CP 0.488057                                                                   DP -0.048522                                                            4     R.sub.y -63.687638                                                                            1.4922 57.50                                                  R.sub.x -52.93087                                                                             Y 42.650                                                                             θ 21.79°                                  K.sub.y 0       Z 25.411                                                      K.sub.x 0                                                                     AR 0.237756 × 10.sup.-5                                                 BR 0.497821 × 10.sup.-15                                                CR -0.279870 × 10.sup.-13                                               DR -0.880162 × 10.sup.-16                                               AP -0.194847                                                                  BP 54.229862                                                                  CP -0.082177                                                                  DP -0.563852                                                            5     R.sub.y -32.35945                                                                             Y 28.650                                                                             θ 71.97°                                  R.sub.x -28.57891                                                                             Z 28.100                                                      K.sub.y 0                                                                     K.sub.x 0                                                                     AR 0.180717 × 10.sup.-4                                                 BR -0.215221 × 10.sup.-7                                                CR -0.107474 × 10.sup.-9                                                DR -0.117215 × 10.sup.-12                                               AP -1.132480                                                                  BP -0.971905                                                                  CP 0.000688                                                                   DP 0.051621                                                             6     ∞    0.250                                                                              1.5163 64.10                                                                  Y 23.040                                                                             θ 47.79°                                                  Z 40.285                                                7     ∞ (DOE)                                                                            0.000                                                                              1001   -3.45                                            8     -464779                                                                       K 0                                                                           A -0.796488 × 10.sup.-8                                                 B 0.233311 × 10.sup.-10                                           9     ∞(display surface)                                                                      Y 27.154                                                                             θ 37.36°                                                  Z 43.842                                                Example 5                                                                     1     ∞(pupil)                                                          2     R.sub.y -292.331787                                                                           1.5163 64.10                                                  R.sub.x -64.47050                                                                             Y 33.943                                                                             θ 7.70°                                   K.sub.y 0       Z 27.360                                                      K.sub.x 0                                                                     AR 0.401020 × 10.sup.-6                                                 AP -0.693944                                                            3     R.sub.y -1501.496515                                                                          1.5163 64.10                                                  R.sub.x -102.59918                                                                            Y 94.211                                                                             θ -10.16°                                 K.sub.y 0       Z 64.769                                                      K.sub.x 0                                                                     AR -0.870453 × 10.sup.-7                                                AP -0.042140                                                            4     R.sub.y -292.331787                                                                           1.5163 64.10                                                  R.sub.x -64.47050                                                                             Y 33.943                                                                             θ 7.70°                                   K.sub.y 0       Z 27.360                                                      K.sub.x 0                                                                     AR 0.401020 × 10.sup.-6                                                 AP -0.693944                                                            5     R.sub.y -119.796883                                                                           1.5163 64.10                                                  R.sub.x -49.36833                                                                             Y 63.187                                                                             θ 38.77°                                  K.sub.y 0       Z 19.430                                                      K.sub.x 0                                                                     AR 0.946153 × 10.sup.-6                                                 AP -0.548848                                                            6     R.sub.y -292.331787                                                                           1.5163 64.10                                                  R.sub.x -64.47050                                                                             Y 33.943                                                                             θ 7.70°                                   K.sub.y 0       Z 27.360                                                      K.sub.x 0                                                                     AR 0.401020 × 10.sup.-6                                                 AP -0.693944                                                            7     R.sub.y -119.796883                                                                           Y 63.187                                                                             θ 38.77°                                  R.sub.x -49.36833                                                                             Z 19.430                                                      K.sub.y 0                                                                     K.sub.x 0                                                                     AR 0.946153 × 10.sup.-6                                                 AP -0.548848                                                            8     ∞    1.000                                                                              1.5163 64.10                                                                  Y 40.627                                                                             θ 30.59°                                                  Z 35.332                                                9     ∞ (DOE)                                                                            0.000                                                                              1001   -3.45                                            10    -43047.588167                                                                 K 0                                                                           A 0.436299 × 10.sup.-8                                            11    ∞(display surface)                                                                      Y 47.9 θ 27.12°                                                  Z 40                                                    Example 6                                                                     1     ∞(pupil)                                                          2     ∞    0.300                                                                              1.5163 64.10                                                                  Y 0.528                                                                              θ 3.52°                                                   Z 30.968                                                3     ∞ (DOE)                                                                            0.000                                                                              1001   -3.45                                            4     -1014751.75463                                                                K 0                                                                           A 0.787268 × 10.sup.9                                                   B 0.182929 × 10.sup.11                                                  C -0.131126 × 10.sup.-13                                                D 0.130609 × 10.sup.-16                                           5     R.sub.y -73.944129                                                                            1.4922 57.50                                                  R.sub.x -69.91809                                                                             Y 46.027                                                                             θ 24.71°                                  K.sub.y 0       Z 22.895                                                      K.sub.x 0                                                                     AR 0.173092 × 10.sup.-5                                                 BR 0.394778 × 10.sup.-15                                                CR -0.310330 × 10.sup.-13                                               DR -0.421275 × 10.sup.-16                                               AP -0.180837                                                                  BP 56.720372                                                                  CP -0.033823                                                                  DP -0.622396                                                            6     R.sub.y -80.26792                                                                             1.4922 57.50                                                  R.sub.x -62.52811                                                                             Y 17.982                                                                             θ -6.91°                                  K.sub.y 0       Z 47.566                                                      K.sub.x 0                                                                     AR -0.406113 × 10.sup.-12                                               BR 0.137027 × 10.sup.-10                                                CR -0.478711 × 10.sup.-13                                               DR -0.107178 × 10.sup.-15                                               AP -640.994824                                                                BP 2.330409                                                                   CP 0.475262                                                                   DP 0.169588                                                             7     R.sub.y -73.944129                                                                            1.4922 57.50                                                  R.sub.x -69.91809                                                                             Y 46.027                                                                             θ 24.71°                                  K.sub.y 0       Z 22.895                                                      K.sub.x 0                                                                     AR 0.173092 × 10.sup.-5                                                 BR 0.394778 × 10.sup.-15                                                CR -0.310330 × 10.sup.-13                                               DR -0.421275 × 10.sup.-16                                               AP -0.180837                                                                  BP 56.720372                                                                  CP -0.033823                                                                  DP -0.622396                                                            8     R.sub.y -53.640971                                                                            Y 28.627                                                                             θ 83.27°                                  R.sub.x -36.56938                                                                             Z 20.211                                                      K.sub.y 0                                                                     K.sub.x 0                                                                     AR 0.313093 × 10.sup.-4                                                 BR -0.981865 × 10.sup.-7                                                CR 0.153577 × 10.sup.-9                                                 DR -0.710929 × 10.sup.-13                                               AP -0.887541                                                                  BP -0.531847                                                                  CP -0.347911                                                                  DP -0.245694                                                            9     ∞(display surface)                                                                      Y 27.588                                                                             θ 43.23°                                                  Z 43.381                                                Example 7                                                                     1     ∞(pupil)                                                          2     ∞         1.4922 57.50                                                                  Y 0    θ 1.79°                                                   Z 31                                                    3     R.sub.y -141.407945                                                                           1.4922 57.50                                                  R.sub.x -126.83786                                                                            Y -5.513                                                                             θ -22.10°                                 K.sub.y 0       Z 43.891                                                      K.sub.x 0                                                                     AR 0.514241 × 10.sup.-6                                                 BR 0.691483 × 10.sup.-10                                                CR 0.237472 × 10.sup.-12                                                DR -0.978814 × 10.sup.-16                                               AP -0.079009                                                                  BP 0.169964                                                                   CP -0.300765                                                                  DP -0.242091                                                            4     ∞         1.4922 57.50                                                                  Y 0    θ 1.79°                                                   Z 31                                                    5     -41.519166      Y 23.703                                                                             θ 44.32°                                                  Z 43.596                                                6     ∞    1.000                                                                              1.5163 64.10                                                                  Y 32.575                                                                             θ 28.99°                                                  Z 51.228                                                7     ∞ (DOE)                                                                            0.000                                                                              1001   -3.45                                            8     91082.1383941                                                                 K 0                                                                           A -0.201624 × 10.sup.-6                                                 B 0.177572 × 10.sup.-8                                                  C -0.722700 × 10.sup.-11                                                D 0.108209 × 10.sup.-13                                           9     ∞(display surface)                                                                      Y 50.265                                                                             θ 24.87°                                                  Z 47.812                                                Example 8                                                                     1     ∞(pupil)                                                          2     ∞         1.58875                                                                              64.1                                                                   Y 0.000                                                                              θ 0.00°                                                   Z 27.000                                                3     ∞ (DOE)   1001.00000                                                                           -3.45                                                                  Y 0.000                                                                              θ 0.00°                                                   Z 28.000                                                4     -2.2939 × 10.sup.+6                                                                     Y 0.000                                                                              θ 0.00°                                                   Z 28.000                                                5     R.sub.y -209.268                                                                              1.49216                                                                              57.5                                                   R.sub.x -95.115 Y 18.335                                                                             θ 12.00°                                  K.sub.y 0.000000                                                                              Z 27.921                                                      K.sub.x 0.000000                                                              AR 0.783868 × 10.sup.-6                                                 BR 0.299472 × 10.sup.-12                                                CR 0.152974 × 10.sup.-13                                                DR -0.502892 × 10.sup.-16                                               AP -0.449899                                                                  BP -0.794713 × 10.sup.+1                                                CP 0.654541                                                                   DP -0.138730                                                            6     R.sub.y -67.801 1.49216                                                                              57.5                                                   R.sub.x -58.220 Y -9.356                                                                             θ -27.44°                                 K.sub.y 0.000000                                                                              Z 38.348                                                      K.sub.x 0.000000                                                              AR 0.427047 × 10.sup.-6                                                 BR -0.770285 × 10.sup.-10                                               CR 0.407932 × 10.sup.-21                                                DR 0.105909 × 10.sup.-16                                                AP 0.107024                                                                   BP 0.496744                                                                   CP 0.119380 × 10.sup.+3                                                 DP -0.923056 × 10.sup.-2                                          7     R.sub.y -209.268                                                                              1.49216                                                                              57.5                                                   R.sub.x -95.115 Y 18.335                                                                             θ 12.00°                                  K.sub.y 0.000000                                                                              Z 27.921                                                      K.sub.x 0.000000                                                              AR 0.783868 × 10.sup.-6                                                 BR 0.299472 × 10.sup.-12                                                CR 0.152974 × 10.sup.-13                                                DR -0.502892 × 10.sup.-16                                               AP -0.449899                                                                  BP -0.794713 × 10.sup.+1                                                CP 0.654541                                                                   DP -0.138730                                                            8     ∞         Y 27.164                                                                             θ 62.56°                                                  Z 27.921                                                9     ∞(display surface)                                                                      Y 27.796                                                                             θ 46.89°                                                  Z 39.073                                                Example 9                                                                     1     ∞(pupil)                                                          2     R.sub.y -209.268                                                                              1.49216                                                                              57.5                                                   R.sub.x -95.115 Y 18.335                                                                             θ 12.00°                                  K.sub.y 0.000000                                                                              Z 27.921                                                      K.sub.x 0.000000                                                              AR 0.783868 × 10.sup.-6                                                 BR 0.299472 × 10.sup.-12                                                CR 0.152974 × 10.sup.-13                                                DR -0.502892 × 10.sup.-16                                               AP -0.449899                                                                  BP -0.794713 × 10.sup.+1                                                CP 0.654541                                                                   DP -0.138730                                                            3     R.sub.y -67.801 1.49216                                                                              57.5                                                   R.sub.x -58.220 Y -9.356                                                                             θ -27.44°                                 K.sub.y 0.000000                                                                              Z 38.348                                                      K.sub.x 0.000000                                                              AR 0.427047 × 10.sup.-6                                                 BR -0.770285 × 10.sup.-10                                               CR 0.407932 × 10.sup.-21                                                DR 0.105909 × 10.sup.-16                                                AP 0.107024                                                                   BP 0.496744                                                                   CP 0.119380 × 10.sup.+3                                                 DP -0.923056 × 10.sup.-2                                          4     R.sub.y -209.268                                                                              1.49216                                                                              57.5                                                   R.sub.x -95.115 Y 18.335                                                                             θ 12.00°                                  K.sub.y 0.000000                                                                              Z 27.921                                                      K.sub.x 0.000000                                                              AR 0.783868 × 10.sup.-6                                                 BR 0.299472 × 10.sup.-12                                                CR 0.1529474 × 10.sup.-13                                               DR -0.502892 × 10.sup.-16                                               AP -0.449899                                                                  BP -0.794713 × 10.sup.+1                                                CP 0.654541                                                                   DP -0.138730                                                            5     ∞         Y 27.164                                                                             θ 62.56°                                                  Z 27.921                                                6     ∞    1.000                                                                              1.58875                                                                              64.1                                                                   Y 25.000                                                                             θ 60.00°                                                  Z 36.000                                                7     ∞ (DOE)                                                                            0.000                                                                              1001.00000                                                                           -3.45                                            8     -0.344807 × 10.sup.+6                                             9     ∞(display surface)                                                                      Y 28.388                                                                             θ 46.88°                                                  Z 39.413                                                Example 10                                                                    1     ∞(pupil)                                                          2     R.sub.y -209.268                                                                              1.49216                                                                              57.5                                                   R.sub.x -95.115 Y 18.335                                                                             θ 12.00°                                  K.sub.y 0.000000                                                                              Z 27.921                                                      K.sub.x 0.000000                                                              AR 0.783868 × 10.sup.-6                                                 BR 0.299472 × 10.sup.-12                                                CR 0.152974 × 10.sup.-13                                                DR -0.502892 × 10.sup.-16                                               AP -0.449899                                                                  BP -0.794713 × 10.sup.+1                                                CP 0.654541                                                                   DP -0.138730                                                            3     R.sub.y -67.801 1.49216                                                                              57.5                                                   R.sub.x -58.220 Y -9.356                                                                             θ -27.44°                                 K.sub.y 0.000000                                                                              Z 38.348                                                      K.sub.x 0.000000                                                              AR 0.427047 × 10.sup.-6                                                 BR -0.770285 × 10.sup.-10                                               CR 0.407932 × 10.sup.-21                                                DR 0.105909 × 10.sup.-16                                                AP 0.107024                                                                   BP 0.496744                                                                   CP 0.119380 × 10.sup.+3                                                 DP -0.923056 × 10.sup.-2                                          4     R.sub.y -209.268                                                                              1.49216                                                                              57.5                                                   R.sub.x -95.115 Y 18.335                                                                             θ 12.00°                                  K.sub.y 0.000000                                                                              Z 27.921                                                      K.sub.x 0.000000                                                              AR 0.783868 × 10.sup.-6                                                 BR 0.299472 × 10.sup.-12                                                CR 0.1529474 × 10.sup.-13                                               DR -0.502892 × 10.sup.-16                                               AP -0.449899                                                                  BP -0.794713 × 10.sup.+1                                                CP 0.654541                                                                   DP -0.138730                                                            5     ∞         Y 25.000                                                                             θ 62.56°                                                  Z 27.921                                                6     ∞ (GRIN)                                                                           5.000                                                                              1.49216                                                                              57.5                                                                          N.sub.1d -0.1005 × 10.sup.-3                                            N.sub.1F -0.1068 × 10.sup.-3                                            N.sub.1C -0.9399 × 10.sup.-4                                     Y 21.500                                                                             θ 62.56°                                                  Z 35.222                                                7     ∞                                                                 8     ∞(display surface)                                                                      Y 29.022                                                                             θ 46.89°                                                  Z 39.742                                                Example 11                                                                    1     ·(pupil)                                                       2     ∞ (GRIN)  1.49216                                                                              57.5                                                                          N.sub.1d -0.1005 × 10.sup.-3                                            N.sub.1F -0.1068 × 10.sup.-3                                            N.sub.1C -0.9399 × 10.sup.-4                                     Y 0.000                                                                              θ 0.00°                                                   Z 25.000                                                3     ∞         Y 0.000                                                                              θ 0.00°                                                   Z 29.000                                                4     R.sub.y -209.268                                                                              1.49216                                                                              57.5                                                   R.sub.x -95.115 Y 18.335                                                                             θ 12.00°                                  K.sub.y 0.000000                                                                              Z 27.921                                                      K.sub.x 0.000000                                                              AR 0.783868 × 10.sup.-6                                                 BR 0.299472 × 10.sup.-12                                                CR 0.152974 × 10.sup.-13                                                DR -0.502892 × 10.sup.-16                                               AP -0.449899                                                                  BP -0.794713 × 10.sup.+1                                                CP 0.654541                                                                   DP -0.138730                                                            5     R.sub.y -67.801 1.49216                                                                              57.5                                                   R.sub.x -58.220 Y -9.356                                                                             θ -27.44°                                 K.sub.y 0.000000                                                                              Z 38.348                                                      K.sub.x 0.000000                                                              AR 0.427047 × 10.sup.-6                                                 BR -0.770285 × 10.sup.-10                                               CR 0.407932 × 10.sup.-21                                                DR 0.105909 × 10.sup.-16                                                AP 0.107024                                                                   BP 0.496744                                                                   CP 0.119380 × 10.sup.+3                                                 DP -0.923056 × 10.sup.-2                                          6     R.sub.y -209.268                                                                              1.49216                                                                              57.5                                                   R.sub.x -95.115 Y 18.335                                                                             θ 12.00°                                  K.sub.y 0.000000                                                                              Z 27.921                                                      K.sub.x 0.000000                                                              AR 0.783868 × 10.sup.-6                                                 BR 0.299472 × 10.sup.-12                                                CR 0.152974 × 10.sup.-13                                                DR -0.502892 × 10.sup.-16                                               AP -0.449899                                                                  BP -0.794713 × 10.sup.+1                                                CP 0.654541                                                                   DP -0.138730                                                            7     ∞         Y 25.000                                                                             θ 62.56°                                                  Z 27.921                                                8     ∞(display surface)                                                                      Y 28.166                                                                             θ 46.89°                                                  Z 39.360                                                Example 12                                                                    1     ∞(pupil)                                                          2     R.sub.y -209.268                                                                              1.49216                                                                              57.5                                                   R.sub.x -95.115 Y 18.335                                                                             θ 12.00°                                  K.sub.y 0.000000                                                                              Z 27.921                                                      K.sub.x 0.000000                                                              AR 0.783868 × 10.sup.-6                                                 BR 0.299472 × 10.sup.-12                                                CR 0.152974 × 10.sup.-13                                                DR -0.502892 × 10.sup.-16                                               AP -0.449899                                                                  BP -0.794713 × 10.sup.+1                                                CP 0.654541                                                                   DP -0.138730                                                            3     R.sub.y -67.801 1.49216                                                                              57.5                                                   R.sub.x -58.220 Y -9.356                                                                             θ -27.44°                                 K.sub.y 0.000000                                                                              Z 38.348                                                      K.sub.x 0.000000                                                              AR 0.427047 × 10.sup.-6                                                 BR -0.770285 × 10.sup.-10                                               CR 0.407932 × 10.sup.-21                                                DR 0.105909 × 10.sup.-16                                                AP 0.107024                                                                   BP 0.496744                                                                   CP 0.119380 × 10.sup.+3                                                 DP -0.923056 × 10.sup.-2                                          4     R.sub.y -209.268                                                                              1.49216                                                                              57.5                                                   R.sub.x -95.115 Y 18.335                                                                             θ 12.00°                                  K.sub.y 0.000000                                                                              Z 27.921                                                      K.sub.x 0.000000                                                              AR 0.783868 × 10.sup.-6                                                 BR 0.299472 × 10.sup.-12                                                CR 0.152974 × 10.sup.-13                                                DR -0.502892 × 10.sup.-16                                               AP -0.449899                                                                  BP -0.794713 × 10.sup.+1                                                CP 0.654541                                                                   DP -0.138730                                                            5     ∞         Y 25.000                                                                             θ 62.56°                                                  Z 27.921                                                6     54.928 (GRIN)                                                                            5.000                                                                              1.49216                                                                              57.5                                                                          N.sub.1d -0.1005 × 10.sup.-3                                            N.sub.1F -0.1068 × 10.sup.-3                                            N.sub.1C -0.9399 × 10.sup.-4                                     Y 21.500                                                                             θ 62.56°                                                  Z 35.222                                                7     84.049                                                                  8     ∞(display surface)                                                                      Y 28.637                                                                             θ 46.89°                                                  Z 39.508                                                Example 13                                                                    1     ∞(pupil)                                                          2     R.sub.y -209.268                                                                              1.49216                                                                              57.5                                                   R.sub.x -95.115 Y 18.335                                                                             θ 12.00°                                  K.sub.y 0.000000                                                                              Z 27.921                                                      K.sub.x 0.000000                                                              AR 0.783868 × 10.sup.-6                                                 BR 0.299472 × 10.sup.-12                                                CR 0.152974 × 10.sup.-13                                                DR -0.502892 × 10.sup.-16                                               AP -0.449899                                                                  BP -0.794713 × 10.sup.+1                                                CP 0.654541                                                                   DP -0.138730                                                            3     R.sub.y -67.801 1.49216                                                                              57.5                                                   R.sub.x -58.220 Y -9.356                                                                             θ -27.44°                                 K.sub.y 0.000000                                                                              Z 38.348                                                      K.sub.x 0.000000                                                              AR 0.427047 × 10.sup.-6                                                 BR -0.770285 × 10.sup.-10                                               CR 0.407932 × 10.sup.-21                                                DR 0.105909 × 10.sup.-16                                                AP 0.107024                                                                   BP 0.496744                                                                   CP 0.119380 × 10.sup.+3                                                 DP -0.923056 × 10.sup.-2                                          4     R.sub.y -209.268                                                                              1.49216                                                                              57.5                                                   R.sub.x -95.115 Y 18.335                                                                             θ 12.00°                                  K.sub.y 0.000000                                                                              Z 27.921                                                      K.sub.x 0.000000                                                              AR 0.783868 × 10.sup.-6                                                 BR 0.299472 × 10.sup.-12                                                CR 0.152974 × 10.sup.-13                                                DR -0.502892 × 10.sup.-16                                               AP -0.449899                                                                  BP -0.794713 × 10.sup.+1                                                CP 0.654541                                                                   DP -0.138730                                                            5     ∞         Y 25.000                                                                             θ 62.56°                                                  Z 27.921                                                6     -52.463 (GRIN)                                                                           5.000                                                                              1.49216                                                                              57.5                                                                          N.sub.1d -0.1005 × 10.sup.-3                                            N.sub.1F -0.1068 × 10.sup.-3                                            N.sub.1C -0.9399 × 10.sup.-4                                     Y 24.263                                                                             θ 62.56°                                                  Z 34.395                                                7     -41.145                                                                 8     ∞(display surface)                                                                      Y 29.137                                                                             θ 46.89°                                                  Z 39.882                                                __________________________________________________________________________

In the above-described examples, particularly Examples 1 to 7, thevalues of the focal length f of the entire optical system, the focallength f of the diffraction optical element, θ₁ in the condition (4), θ₂in the condition (5), d in the condition (6), α in the condition (7) andF in the condition (8) are as follows:

    ______________________________________                                                     f of entire                                                                              f of diffraction                                      Example      optical system                                                                           optical element                                       ______________________________________                                        1            27.715 mm   244.948 mm                                           2            27.715 mm   259.967 mm                                           3            27.286 mm  1210.471 mm                                           4            28.775 mm   464.779 mm                                           5            36.201 mm   43.047 mm                                            6            27.778 mm  1014.751 mm                                           7            38.531 mm   -91.082 mm                                           ______________________________________                                        Example                                                                              Θ.sub.1 (°)                                                                 Θ.sub.2 (°)                                                              d (mm)  α (°)                                                                 F                                      ______________________________________                                        1      95.047     44.658  -0.248  -1.69                                                                              0.11315                                2      93.263     42.891  -5.867  3.89 0.10661                                3      96.631     40.897  -4.410  -6.26                                                                              0.02254                                4      94.909     45.589  0.026   1.07 0.06191                                5      128.768    60.972  2.933   8.07 0.84096                                6      96.191     43.475  -0.542  -3.49                                                                              0.02737                                7      83.442     40.511  -0.037  13.23                                                                              -0.42304                               ______________________________________                                    

Although in the above-described examples anamorphic surfaces are used assurfaces that constitute an optical member, it is also possible to usetoric surfaces, rotationally symmetric aspherical surfaces, sphericalsurfaces, or three-dimensional surfaces (free-form surfaces) which areexpressed by ##EQU3##

where x, y and z denote orthogonal coordinates, C_(nm) is an arbitrarycoefficient, and k and k' are also arbitrary values, respectively.

Further, conditions for the curvature, power, etc. of a surface can alsobe obtained from a curvature in an arbitrary region which is obtainedfrom a surface configuration of a portion of the surface which is struckby axial light rays traveling on the visual axis to reach the imagedisplay device, along the axial light rays.

If a system is arranged by using a pair of optical systems according toeach example, it is possible to project a pair of images into both eyesof the observer.

Although the above-described Examples 1 to 4, 6, and 7 to 13 basicallyuse the optical member 7 shown in FIG. 14, it should be noted thatoptical members 7 such as those shown in FIGS. 15 to 20 may also be usedin combination with a diffraction optical element. In the case of FIG.15, the optical member 7 has a first surface 3, a second surface 4, athird surface 5, and a fourth surface 9. A bundle of light rays emittedfrom the image display device 6 enters the optical member 7 while beingrefracted by the third surface 5. The incident ray bundle is internallyreflected by the fourth surface 9 and then internally reflected by thesecond surface 4 so as to be incident on the first surface 3. The raybundle is refracted by the first surface 3 and projected into theobserver's eyeball with the observer's iris position or eyeball rollingcenter as the exit pupil 1.

In the case of FIG. 16, the optical member 7 has a first surface 3, asecond surface 4, a third surface 5, and a fourth surface 9. A bundle oflight rays emitted from the image display device 6 enters the opticalmember 7 while being refracted by the third surface 5. The incident raybundle is internally reflected by the fourth surface 9 so as to beincident on the third surface 5. This time, the ray bundle is internallyreflected by the third surface 5. Then, the ray bundle is internallyreflected by the second surface 4 so as to be incident on the firstsurface 3. The ray bundle is refracted by the first surface 3 andprojected into the observer's eyeball with the observer's iris positionor eyeball rolling center as the exit pupil 1.

In the case of FIG. 17, the optical member 7 has a first surface 3, asecond surface 4, a third surface 5, and a fourth surface 9. A bundle oflight rays emitted from the image display device 6 enters the opticalmember 7 while being refracted by the third surface 5. The incident raybundle is internally reflected by the second surface 4 and theninternally reflected by the fourth surface 9 so as to be incident on thesecond surface 4 again. The ray bundle is internally reflected by thesecond surface 4 so as to be incident on the first surface 3. The raybundle is refracted by the first surface 3 and projected into theobserver's eyeball with the observer's iris position or eyeball rollingcenter as the exit pupil 1.

In the case of FIG. 18, the optical member 7 has a first surface 3, asecond surface 4, a third surface 5, and a fourth surface 9. A bundle oflight rays emitted from the image display device 6 enters the opticalmember 7 while being refracted by the second surface 4. The incident raybundle is internally reflected by the third surface 5 and theninternally reflected by the second surface 4. The reflected ray bundleis then internally reflected by the fourth surface 9 so as to beincident on the second surface 4 again. The ray bundle is internallyreflected by the second surface 4 so as to be incident on the firstsurface 3. The ray bundle is refracted by the first surface 3 andprojected into the observer's eyeball with the observer's iris positionor eyeball rolling center as the exit pupil 1.

In the case of FIG. 19, the optical member 7 is similar to that inExample 5. A bundle of light rays emitted from the image display device6 enters the optical member 7 while being refracted by the third surface5. The incident ray bundle is internally reflected by the first surface3 so as to be incident on the third surface 5 again. This time, the raybundle is internally reflected by the third surface 5 and theninternally reflected by the first surface 3. The reflected ray bundle isreflected by the second surface 4 so as to be incident on the firstsurface 3 once again. The ray bundle is refracted by the first surface 3and projected into the observer's eyeball with the observer's irisposition or eyeball rolling center as the exit pupil 1.

In the case of FIG. 20, a bundle of light rays emitted from the imagedisplay device 6 enters the optical member 7 while being refracted bythe first surface 3. The incident ray bundle is internally reflected bythe third surface 5 so as to be incident on the first surface 3 again.This time, the ray bundle is internally reflected by the first surface 3and then internally reflected by the third surface 5 again so as to beincident on the first surface 3 once again. The ray bundle is internallyreflected by the first surface 3 and then reflected by the secondsurface 4 so as to be incident on the first surface 3 once again. Theray bundle is refracted by the first surface 3 and projected into theobserver's eyeball with the observer's iris position or eyeball rollingcenter as the exit pupil 1.

Thus, in the optical system according to the present invention, a DOE ora GRIN is used to correct chromatic aberration, field curvature andother aberrations remaining uncorrected in a single decentered prismwhich has three or four optical surfaces and in which a space formedbetween these optical surfaces is filled with a medium having arefractive index larger than 1. When the optical system is used as anocular optical system, the arrangement is not necessarily limited to onethat is designed for a single eye. It is also possible to use a pair ofoptical systems arranged according to the present invention. If a pairof optical systems are used to enable observation with both eyes, it ispossible for the observer to see the observation image without fatigue.Further, if images with a disparity therebetween are presented to botheyes, it is possible to view the observation image as a stereoscopicimage. If a pair of ocular optical systems according to the presentinvention are used in combination with a mechanism for supporting themon the observer's head, it becomes possible for the observer to see theobservation image in his/her own easy posture.

Incidentally, the problem of flare or ghost light is associated with animage display apparatus that uses an ocular optical system 10comprising, for example, a decentered prism which has two to fouroptical surfaces and in which a space formed by the optical surfaces isfilled with a medium having a refractive index larger than 1, asdescribed above. More specifically, light from the image display device6 which does not participate in display may enter the observer's eyeballdirectly or by being irregularly reflected, thus forming flare light orghost light. Further, external light may enter the observer's eyeball bybeing reflected by some surface of the ocular optical system 10, thusforming flare light or ghost light. Such flare or ghost light causes thedisplayed image quality to be degraded. This will be briefly explainedbelow by taking the optical system shown in FIG. 14 as an example.

FIG. 44(a) shows an optical path of normal display light. In the case ofthe ocular optical system 10 shown in FIG. 14, display light from theimage display device 6, e.g. an LCD (Liquid Crystal Display), enters theocular optical system 10 through the first surface 5 and is totallyreflected by the fourth surface, which is formed by the third surface 3serving as both transmitting and reflecting surfaces. The reflectedlight is then reflected by the second surface 4 and exits from theocular optical system 10 through the third surface 3 to enter theobserver's pupil 1, thereby allowing an image displayed by the imagedisplay device 6 to be viewed as an enlarged image. FIG. 44(b) shows anoptical path of a first type of ghost light. A part of light from theimage display device 6 enters the pupil 1 directly through the firstsurface 5 and fourth surface (third surface 3) of the ocular opticalsystem 10 as ghost light, thus undesirably forming a ghost or otherunwanted image outside the display area. FIG. 44(c) shows an opticalpath of a second type of ghost light. A part of light from the imagedisplay device 6 enters the ocular optical system 10 through the firstsurface 5 and is reflected by the fourth surface (third surface 3) so asto return to the first surface 5. The reflected light is reflected atthe back of the first surface 5 and then enters the pupil 1 via thesecond surface 4 sand the third surface 3 as ghost light, as shown inthe figure, thus similarly forming a ghost or other unwanted imageoutside the display area. FIG. 44(d) shows an optical path of a thirdtype of ghost light. In this case, external light from the outside worldis reflected by some surface (the second surface 4 in the case of theillustrated example) constituting the ocular optical system 10 so as toenter the pupil 1 as flare or ghost light, causing the displayed imagequality to be degraded.

FIGS. 24(a) to 24(d) are views for explanation of optical paths in animage display apparatus according to Example 14 of the presentinvention, which correspond to FIGS. 44(a) to 44(d). That is, FIG. 24(a)shows an optical path of normal display light, and FIGS. 24(b), 24(c)and 24(d) show optical paths of the first, second and third types ofghost light, respectively. Although the ocular optical system 10 shownin FIG. 14 is used in this example, the same is true of the ocularoptical systems 10 shown in FIGS. 15 to 20. In this example, alight-blocking plate 31 of light absorption properties, which isprovided with a horizontally elongate rectangular aperture 32, isdisposed between the ocular optical system 10 and the observer's pupil1, thereby preventing flare or ghost light from entering the pupil 1.

More specifically, in the case of normal display light in FIG. 24(a),display light from the image display device 6, e.g. an LCD, enters theocular optical system 10 through the first surface 5 and is totallyreflected by the fourth surface, which is formed by the third surface 3serving as both transmitting and reflecting surfaces. Then, thereflected light is reflected by the second surface 4 and exits from theocular optical system 10 through the third surface 3 to enter theobserver's pupil 1 through the aperture 32 without being blocked by thelight-blocking plate 31, thus enabling an image displayed by the imagedisplay device 6 to be viewed as an enlarged image. In the case of thefirst type of ghost light shown in FIG. 24(b), light from the imagedisplay device 6 passes directly through the first surface 5 and fourthsurface (third surface 3) of the ocular optical system 10, but it isprevented from entering the pupil 1 by the light-blocking plate 31because the angle of incidence on the pupil 1 is larger than the fieldangle of the display area. Accordingly, no ghost light is introduced. Inthe case of the second type of ghost light shown in FIG. 24(c), lightfrom the image display device 6 enters the ocular optical system 10through the first surface 5 to reach the fourth surface (the thirdsurface 3). The light is reflected by the fourth surface to return tothe first surface 5 and then reflected at the back of the first surface5 so as to exit from the ocular optical system 10 via the second surface4 and the third surface 3, as shown in the figure. In this case also,the light is prevented from entering the pupil 1 by the light-blockingplate 31 because the angle of incidence on the pupil 1 is larger thanthe field angle of the display area. Accordingly, no ghost light isintroduced. The third type of ghost light shown in FIG. 24(d) isexternal light from the outside world. Such external light is blocked bythe light-blocking plate 31 of light absorption properties and cannotenter the ocular optical system 10. Therefore, no external light isreflected by any surface of the ocular optical system 10. Accordingly,neither flare nor ghost light is produced.

Thus, by disposing the light-blocking plate 31 having the aperture 32between the ocular optical system 10 and the observer's pupil 1, it ispossible to block flare and ghost light without intercepting displaylight from the image display device 6, and hence possible to effectivelyprevent degradation of the displayed image quality due to flare or ghostlight based on light from the image display device 6 and light from theoutside world.

Let us discuss the size of the aperture 32 in the light-blocking plate31 that enables flare and ghost light to be effectively blocked. With aview to effectively blocking the three types of flare and ghost light,shown in FIGS. 44(a) to 44(d) without eclipsing normal display light, itis desirable to satisfy the following condition:

    Ltanθ+.o slashed./2≦R≦2 (Ltanθ+.o slashed./2)(22)

where, as shown in FIG. 25, R is the distance from the edge of theaperture 32 in the light-blocking plate 31 to the visual axis 2 in asection containing the center of the image display device 6 and thevisual axis 2, .o slashed. is the diameter of the exit pupil of theocular optical system 10, L is the eye relief of the ocular opticalsystem 10, and θ is the half view angle in the section containing thecenter of the image display device 6 and the visual axis 2.

It is more desirable to satisfy the following condition:

    Ltanθ+.o slashed./2≦R≦1.2 (Ltanθ+.o slashed./2)(22')

FIGS. 26 to 37 each show the way in which the same light-blocking plate31 is disposed in another type of ocular optical system 10 and theobserver's pupil 1. Ocular optical systems 10 shown in FIGS. 28 to 31correspond to the optical systems shown in FIGS. 15 to 18; therefore,description thereof is omitted. The operation of the light-blockingplate 31 will, however, be clear from the foregoing description. Let usgive a brief explanation of ocular optical systems 10 shown in FIGS. 26,27 and 32 to 37. In these ocular optical systems 10, the operation ofthe light-blocking plate 31 will be clear from the foregoingdescription.

The ocular optical system 10 shown in FIG. 26 is a decentered prismwhich has four optical surfaces 3, 4, 5 and 9 and in which a spaceformed between these optical surfaces is filled with a medium having arefractive index larger than 1. Display light from the image displaydevice 6 enters the ocular optical system 10 through the first surface5, which is a transmitting surface disposed to face the image displaydevice 6. The incident light is reflected by the fourth surface 9, whichis a decentered reflecting surface. The reflected light is reflected bythe second surface 4, which is a decentered reflecting surface disposedon the observer's visual axis 2 to face the observer's pupil 1. Thereflected light intersects the light incident on the fourth surface 9and exits from the ocular optical system 10 through the third surface 3,which is a transmitting surface disposed on the observer's visual axis 2between the second surface 4 and the observer's pupil 1. Then, the lighttravels along the observer's visual axis 2, enters the observer's pupil1 without forming an intermediate image, and forms an image on theobserver's retina.

The ocular optical system 10 shown in FIG. 27 is a decentered prismwhich has three optical surfaces 3 to 5 and in which a space formedbetween these optical surfaces is filled with a medium having arefractive index larger than 1. Display light from the image displaydevice 6 enters the ocular optical system 10 through the first surface5, which is a transmitting surface disposed to face the image displaydevice 6. The incident light is reflected by the second surface 4, whichis a decentered reflecting surface. The reflected light exits from theocular optical system 10 through the third surface 3, which is atransmitting surface disposed on the observer's visual axis 2 betweenthe second surface 4 and the observer's pupil 1. Then, the light travelsalong the observer's visual axis 2, enters the observer's pupil 1without forming an intermediate image, and forms an image on theobserver's retina.

The ocular optical system 10 shown in FIG. 32 is a decentered prismwhich has three optical surfaces 3, 4 and 9 and in which a space formedbetween these optical surfaces is filled with a medium having arefractive index larger than 1. Display light from the image displaydevice 6 enters the ocular optical system 10 through the first surface5, which is a transmitting surface disposed to face the image displaydevice 6. The first surface 5 is formed by the second surface 4, whichserves as both reflecting and transmitting surfaces. The second surface4 is a decentered surface disposed on the observer's visual axis 2 toface the observer's pupil 1. The incident light is reflected by thefifth surface 15, which is a reflecting surface formed by the thirdsurface 3 serving as both transmitting and reflecting surfaces. Thethird surface 3 is disposed on the observer's visual axis 2 between thesecond surface 4 and the observer's pupil 1. The reflected light isreflected by the fourth surface 9, which is a reflecting surface. Thereflected light is reflected by the second surface 4, which is adecentered reflecting surface disposed on the observer's visual axis 2to face the observer's pupil 1. The reflected light intersects the lightreflected by the fifth surface 15 and exits from the ocular opticalsystem 10 through the third surface 3, which is a transmitting surfacedisposed on the observer's visual axis 2 between the second surface 4and the observer's pupil 1. Then, the light travels along the observer'svisual axis 2, enters the observer's pupil 1 without forming anintermediate image, and forms an image on the observer's retina.

The ocular optical system 10 shown in FIG. 33 is a decentered prismwhich has three optical surfaces 3, 4 and 9 and in which a space formedbetween these optical surfaces is filled with a medium having arefractive index larger than 1. Display light from the image displaydevice 6 enters the ocular optical system 10 through the first surface5, which is a transmitting surface disposed to face the image displaydevice 6. The first surface 5 is formed by the second surface 4, whichserves as both reflecting and transmitting surfaces. The second surface4 is a decentered surface disposed on the observer's visual axis 2 toface the observer's pupil 1. The incident light is reflected by thefourth surface 9, which is a reflecting surface. The reflected light isreflected by the second surface 4, which is a decentered reflectingsurface disposed on the observer's visual axis 2 to face the observer'spupil 1. The reflected light exits from the ocular optical system 10through the third surface 3, which is a transmitting surface disposed onthe observer's visual axis 2 between the second surface 4 and theobserver's pupil 1. Then, the light travels along the observer's visualaxis 2, enters the observer's pupil 1 without forming an intermediateimage, and forms an image on the observer's retina.

The ocular optical system 10 shown in FIG. 34 is a decenteredback-coated mirror which has two optical surfaces 4 and 5 and in which aspace formed between these optical surfaces is filled with a mediumhaving a refractive index larger than 1. Display light from the imagedisplay device 6 enters the ocular optical system 10 through the firstsurface 5, which is a transmitting surface disposed to face the imagedisplay device 6. The incident light is reflected by the second surface4, which is a decentered reflecting surface disposed on the observer'svisual axis 2 to face the observer's pupil 1. The reflected light exitsfrom the ocular optical system 10 through the third surface 3, which isa transmitting surface disposed on the observer's visual axis 2 betweenthe second surface 4 and the observer's pupil 1. The third surface 3 isformed by the first surface 5, which serves as two transmittingsurfaces. Then, the light travels along the observer's visual axis 2,enters the observer's pupil 1 without forming an intermediate image, andforms an image on the observer's retina.

The ocular optical system 10 shown in FIG. 35 has a decenteredback-coated mirror 16, and either another back-coated mirror orsurface-coated mirror 17 which is disposed at the entrance side of theback-coated mirror 16. Display light from the image display device 6enters the decentered back-coated mirror 16 after being reflected by theback-coated mirror or surface-coated mirror 17 disposed to face theimage display device 6. The display light enters the back-coated mirror16 through the first surface 5, which is a transmitting surface, and isreflected by the second surface 4, which is a decentered reflectingsurface disposed on the observer's visual axis 2 to face the observer'spupil 1. The reflected light exits from the ocular optical system 10through the third surface 3, which is a transmitting surface disposed onthe observer's visual axis 2 between the second surface 4 and theobserver's pupil 1. The third surface 3 is formed by the first surface5, which serves as two transmitting surfaces. Then, the light travelsalong the observer's visual axis 2, enters the observer's pupil 1without forming an intermediate image, and forms an image on theobserver's retina.

The ocular optical system 10 shown in FIG. 36 has a decenteredback-coated mirror 16, and a refracting lens system 18 which is disposedat the entrance side of the back-coated mirror 16. Display light fromthe image display device 6 enters the decentered back-coated mirror 16through the refracting lens system 18, which is disposed to face theimage display device 6. The display light enters the back-coated mirror16 through the first surface 5, which is a transmitting surface, and isreflected by the second surface 4, which is a decentered reflectingsurface disposed on the observer's visual axis 2 to face the observer'spupil 1. The reflected light exits from the ocular optical system 10through the third surface 3, which is a transmitting surface disposed onthe observer's visual axis 2 between the second surface 4 and theobserver's pupil 1. The third surface 3 is formed by the first surface5, which serves as two transmitting surfaces. Then, the light travelsalong the observer's visual axis 2, enters the observer's pupil 1without forming an intermediate image, and forms an image on theobserver's retina.

The ocular optical system 10 shown in FIG. 37 is a decentered compoundlens which has two optical surfaces 4 and 5 and in which a space formedbetween these optical surfaces is filled with a medium having arefractive index larger than 1. Display light from the image displaydevice 6 enters the ocular optical system 10 through the first surface5, which is a transmitting surface disposed to face the image displaydevice 6. The incident light passes through the second surface 4, whichis a semitransparent surface, and is then reflected by the fourthsurface 9, which is a semitransparent surface. The reflected light isreflected by the second surface 4, which is a decentered semitransparentsurface disposed on the observer's visual axis 2 to face the observer'spupil 1. The reflected light exits from the ocular optical system 10through the third surface 3, which is a transmitting surface disposed onthe observer's visual axis 2 between the second surface 4 and theobserver's pupil 1. Then, the light travels along the observer's visualaxis 2, enters the observer's pupil 1 without forming an intermediateimage, and forms an image on the observer's retina.

It should be noted that the above-described first to sixth surfaces maybe formed from plane surfaces, spherical surfaces, aspherical surfaces,anamorphic surfaces, or anamorphic aspherical surfaces. At least one ofthe surfaces has positive power. Reflection at a surface which serves asboth transmitting and reflecting surfaces, e.g. a surface constitutingboth the third and fourth surfaces in FIG. 14, may be total reflection,or effected by a back-coated mirror in a case where transmitting andreflecting regions are separated from each other.

It should be noted that the light-blocking plate 31 may be provided inclose contact with the pupil-side surface of the ocular optical system10 by coating or other similar method. FIGS. 38(a) to 38(d) are viewscorresponding to FIGS. 24(a) to 24(d). In these figures, thelight-blocking plate 31 having the aperture 32 is so shaped as toconform to the pupil-side surface (i.e. the third surface 3 in thesefigures) of the ocular optical system 10 and stuck fast to that surface.Operations taking place in FIGS. 38(a) to 38(d) are the same as those inFIGS. 24(a) to 24(d); therefore, description thereof is omitted. Thesame is true of the ocular optical systems 10 shown in FIGS. 26 to 37.However, if the light-blocking plate 31 is placed in close contact witha surface of the ocular optical system 10 in a case where totalreflection at that surface is used, the conditions for the totalreflection at the surface are destroyed by the close contact of thelight-blocking plate 31. Therefore, it is necessary to providereflective coating on that surface and to bring the light-blocking plate31 into close contact with the coating.

Incidentally, ghost light, particularly one such as that shown in FIG.44(b), occurs because the angle of divergence of display light from theimage display device 6 (i.e. NA: numerical aperture) is excessivelylarge. Assuming that the focal length of the ocular optical system 10is, for example, 30 mm, and the diameter of the observer's pupil 1, thatis, the exit pupil diameter of the ocular optical system 10, is 4 mm,and that the ocular optical system 10 is approximately telecentric onthe image display device side, the angle of divergence of necessarylight rays, which enter the observer's pupil 1, is 3.8° according to ageneral formula for NA. In other words, among light rays emitted fromthe image display device 6, those which are at 3.8° or more with respectto the normal to the image display device 6 have no effect on the imagefor observation and; rather, they are unwanted light rays and may causeflare or ghost. Accordingly, flare light and ghost light can also bereduced by limiting the NA of display light emitted from the imagedisplay device 6. To limit the NA of display light, as shown in FIG. 39,an NA limiting member 33, e.g. a louver, is disposed between the imagedisplay device 6 and the ocular optical system 10. A louver is apreferred example of the NA limiting member 33. As shown in thesectional view of FIG. 41, a louver 35 comprises light-transmittingopenings 36 and light-blocking (light-absorbing) walls 37, which arealternately arranged in a one- or two-dimensional periodic pattern. Thelouver 35 selectively transmits only light in the range of a narrowangle β in a specific direction. Accordingly, when the louver 35 isdisposed between the image display device 6 and the ocular opticalsystem 10, display light emitted from the image display device 6 at awide divergence angle α is limited to the narrow angle β. Thus, lightemitted from the image display device 6 at a large exit angle, which maycause ghost light, is cut off. Another example of the NA limiting member33 is an optical fiber plate. The operation of the optical fiber plateis the same as that of the louver 35. It is also possible to use a fieldselecting glass as the NA limiting member 33. One example of fieldselecting glass is known as "Angle 21" (trade name; manufactured byNippon Sheet Glass Co., Ltd.), which transmits light entering it at asmall incident angle, but scatters light incident thereon at a largeangle, thereby preventing it from passing therethrough.

It is desirable for the NA limiting member 33 to limit the NA such thatthe angle θ between the normal to the image display device 6 and a lightray extending at the largest divergence angle on the observer's pupilside satisfies the following condition:

    θ<45°                                         (23)

It is more desirable to satisfy the following condition:

    θ<20°                                         (23')

FIG. 40 shows another arrangement for limiting the NA of display lightemitted from the image display device 6. As shown in the figure, when anLCD 6' is used as the image display device 6, an NA limiting member 33such as that described above is disposed between the LCD 6' and anilluminating light source 34 therefor. In this case also, display lightfrom the image display device 6 is limited to a narrow angle, and thereis no light emanating from the image display device 6 at a large exitangle at which ghost light may be produced.

It should be noted that, in the arrangements shown in FIGS. 39 and 40,the angle range of a bundle of transmitted light rays limited by the NAlimiting member 33 need not always be symmetric with respect to thenormal to the image display device 6 or 6'. There are cases where it ispreferable to limit the NA of a ray bundle more at a side opposite tothe observer see FIGS. 43(a) and 43(b)!. In such a case, the principalray of the transmitted ray bundle shifts from the normal to the imagedisplay device 6 or 6'.

When the LCD 6' is used as the image display device 6 in any of theocular optical systems 10 shown in FIGS. 25 to 37, it is possible tolimit the NA of display light entering the ocular optical system 10 soas to prevent occurrence of flare or ghost light by properly selectingthe relative position of the LCD 6' and the illuminating light source34. This scheme will be explained below.

As shown in FIGS. 42(a) to 42(c), the distance between the displaysurface of the image display device 6 and the illuminating light source34 is defined as d, and the angle between the normal to the imagedisplay device 6 and an imaginary straight line connecting an end pointon the light-emitting surface of the illuminating light source 34 andthe corresponding end point on the display surface is defined as θ. Asthe illuminating light source 34 is moved away from the image displaydevice 6 such that the distance d changes to be d₀ <d₁ <d₂ FIGS.42(a)→42(b)→42(c)!, the incident angle θ of light rays from theilluminating light source 34 at the edge of the image display areachanges to be θ₀ >θ₁ >θ₂. That is, as the distance d between the displaysurface of the image display device 6 and the illuminating light source34 increases (d₀ →d₁ →d₂), the incident angle θ of light rays from theilluminating light source 34 at the edge of the image display areadecreases (θ₀ →θ₁ →θ₂).

Accordingly, it is desirable that the distance d between the displaysurface of the image display device 6 and the illuminating light source34 should satisfy the following condition:

    Sb>d>1 mm                                                  (24)

where Sb is the size of the illuminating light source 34.

By setting the distance between the display surface of the image displaydevice 6 and the illuminating light source 34 at 1 mm or more, theincident angle of light rays from the illuminating light source 34 atthe edge of the image display area, that is, the incident angle at whichthe light rays enter the ocular optical system 10, becomessatisfactorily small, as shown in FIGS. 42(a) to 42(c). Thus, it ispossible to reduce unwanted light which causes flare or ghost.

It will be apparent that the above-described purpose can be attained bycombining the following conditions. That is, it is preferable to satisfythe following condition:

    Sb>d>2 mm                                                  (24')

If the distance d between the display surface of the image displaydevice 6 and the illuminating light source 34 is 2 mm or more, theincident angle θ of light rays from the illuminating light source 34 atthe edge of the image display area becomes smaller, and unwanted lightcan be further reduced.

It is more desirable to satisfy the following condition:

    Sb>d>3 mm                                                  (24")

If the condition (24") is satisfied, the incident angle θ of light raysfrom the illuminating light source 34 at the edge of the image displayarea becomes further smaller, and unwanted light can be reduced evenmore effectively. However, because the quantity of light from theilluminating light source 34 reduces, it is preferable to determine anoptimum value by trial and error.

Ghost light such as that shown in FIG. 44(b) is caused by light rays oflarge NA emanating from a position on the display surface of the imagedisplay device 6 which is more away from the position of the observer'spupil 1. Incidentally, assuming that, as shown in FIGS. 43(a) and 43(b),the distance between one end of the illuminating light source 34 and thecorresponding end of the image display device 6 is d_(s), if theilluminating light source 34 is tilted such that d_(s1) <d_(s2) as shownin FIGS. 43(a)→43(b), the incident angle of light rays from theilluminating light source 34 at the edge of the image display area isθ_(s1) >θ_(s2). In other words, as the distance d between theilluminating light source 34 and the image display device 6 increases(d_(s1) →d_(s2)), the incidence angle θ of light rays from theilluminating light source 34 at the edge of the image display areadecreases (θ_(s1) →θ_(s) ₂). Accordingly, it is desirable to tilt theilluminating light source 34 and the image display device 6 relative toeach other such that the illuminating light source 34 and the imagedisplay device 6 diverge from each other at ends thereof which areremote from the observer's pupil 1, and that the tilt angle satisfiesthe following condition:

    Sb>d.sub.s >1 mm                                           (25)

where d_(s) is the distance between the illuminating light source 34 andthe display surface of the image display device 6 at their respectiveends where the illuminating light source 34 and the image display device6 diverge from each other.

By tilting the illuminating light source 34 and the image display device6 relative to each other such that the illuminating light source 34 andthe image display device 6 diverge from each other at ends thereof whichare remote from the observer's eyeball, and that the tilt anglesatisfies the condition (25), the inclination angle θ_(s) of light raysemanating from the edge of the image display area is reduced, as shownin FIG. 43(b). Thus, it is possible to reduce unwanted light such asthat shown in FIG. 44(b).

It is more desirable to satisfy the following condition:

    Sb>d.sub.s >2 mm                                           (25')

By tilting the illuminating light source 34 and the image display device6 relative to each other such that the distance between the illuminatinglight source 34 and the display surface of the image display device 6 atthe diverging ends is 2 mm or more, the inclination angle θ_(s) of lightrays emanating from the edge of the image display area is furtherreduced, and unwanted light can be further reduced.

It is still more desirable to satisfy the following condition:

    Sb>d.sub.s >3 mm                                           (25")

By tilting the illuminating light source 34 and the image display device6 relative to each other such that the distance between the illuminatinglight source 34 and the display surface of the image display device 6 atthe diverging ends is 3 mm or more, the inclination angle θ_(s) of lightrays emanating from the edge of the image display area is still furtherreduced, and unwanted light can be reduced very effectively.

It is possible to form a portable image display apparatus, such as astationary or head-mounted image display apparatus, which enables theobserver to see with both eyes by preparing a pair of combinations of anoptical system according to the present invention, used as an ocularoptical system, and an image display device for the left and right eyes,and supporting them apart from each other by the distance between theeyes. FIG. 45 shows the whole arrangement of an example of such aportable image display apparatus. A display apparatus body unit 50contains a pair of left and right ocular optical systems such as thosedescribed above, and image display devices comprising liquid crystaldisplay devices are disposed at the respective image planes of the twoocular optical systems. The apparatus body unit 50 is provided with apair of left and right temporal frames 51 which are contiguous with theleft and right ends of the apparatus body unit 50, as illustrated in thefigure. The two temporal frames 51 are connected by a top frame 52. Inaddition, a rear frame 54 is attached to the intermediate portion ofeach temporal frame 51 through a leaf spring 53. Thus, by applying therear frames 54 to the rear portions of the observer's ears like thetemples of a pair of glasses and placing the top frame 52 on the top ofthe observer's head, the display apparatus body unit 50 can be held infront of the observer's eyes. It should be noted that a top pad 55,which is an elastic material such as a sponge, is attached to the innerside of the top frame 52, and a similar pad is attached to the innerside of each rear frame 54, thereby allowing the user to wear thedisplay apparatus on his or her head without feeling uncomfortable.

Further, a speaker 56 is provided on each rear frame 54 to enable theuser to enjoy listening to stereophonic sound in addition to imageobservation. The display apparatus body unit 50 having the speakers 56is connected with a reproducing unit 58, e.g. a portable video cassetteunit, through an image and sound transmitting cord 57. Therefore, theuser can enjoy not only observing an image but also listening to soundwith the reproducing unit 58 retained on a desired position, e.g. abelt, as illustrated in the figure. Reference numeral 59 in the figuredenotes a switch and volume control part of the reproducing unit 58. Itshould be noted that the top frame 52 contains electronic parts such asimage and sound processing circuits.

The cord 57 may have a jack and plug arrangement attached to the distalend thereof so that the cord 57 can be detachably connected to anexisting video deck. The cord 57 may also be connected to a TV signalreceiving tuner so as to enable the user to enjoy watching TV.Alternatively, the cord 57 may be connected to a computer to receivecomputer graphic images or message images or the like from the computer.To eliminate the bothersome cord, the image display apparatus may bearranged to receive external radio signals through an antenna connectedthereto.

Further, the optical system according to the present invention can beused as an imaging optical system. For example, as shown in theperspective view of FIG. 46, the optical system may be used in a finderoptical system F_(i) of a compact camera C_(a) in which a photographicoptical system O_(b) and the finder optical system F_(i) are providedseparately in parallel to each other. FIG. 47 shows the arrangement ofan optical system in a case where the present invention is used as suchan imaging optical system. As illustrated, the optical system DSaccording to the present invention is disposed behind a front lens groupGF and an aperture diaphragm D, thereby constituting an objectiveoptical system L_(t). An image that is formed by the objective opticalsystem L_(t) is erected by a Porro prism P, in which there are fourreflections, provided at the observer side of the objective opticalsystem L_(t), thereby enabling an erect image to be observed through anocular lens O_(c).

Although the optical system according to the present invention, togetherwith the image display apparatus using the optical system as an ocularoptical system, has been described above by way of examples, it shouldbe noted that the present invention is not necessarily limited to theseexamples, and that various modifications may be imparted thereto.

As will be clear from the foregoing description, the optical systemaccording to the present invention makes it possible to provide animaging optical system which is compact and lightweight and favorablycorrected for aberrations, and an optical system suitable for use as anocular optical system for a head- or face-mounted image displayapparatus.

Further, the image display apparatus according to the present inventionis arranged such that chromatic aberration, field curvature, etc. whichare produced by transmitting surfaces of a single decentered prism whichhas three or four optical surfaces and in which a space formed betweenthese optical surfaces is filled with a medium having a refractive indexlarger than 1 are corrected by disposing a correction optical element ata position between the image display device and the observer's pupilsuch that the correction optical member produces aberrations opposite insign to aberrations produced by the transmitting surfaces of thedecentered prism. As the correction optical element, a DOE or a gradientindex lens, which are useful for correction of the above-describedaberrations, is used, thereby enabling residual aberrations,particularly chromatic aberration, to be satisfactorily corrected evenwhen the pixel density of the image display device increases. Thus, itis possible to obtain a head- or face-mounted image display apparatuswhich has a compact size and a wide field angle and is capable ofdisplaying a clear image over the entire image field.

What we claim is:
 1. An image display apparatus comprising:an imagedisplay device that displays an image; an ocular optical system thatleads the image displayed by said image display device to an observer'seyeball without forming an intermediate real image; and a retainingcomponent that retains both said image display device and said ocularoptical system on an observer's head or face, wherein said image displayapparatus includes a light-blocking member provided between said ocularoptical system and said observer's eyeball, and said ocular opticalsystem has three to four optical surfaces, and an area surrounded bysaid optical surfaces is filled with a transparent medium having arefractive index larger than 1, said optical surfaces including a firstsurface disposed to face said image display device, a second surfacedisposed on an observer's visual axis to face an observer's pupil, and athird surface disposed on said observer's visual axis between saidsecond surface and said observer's pupil.
 2. An image display apparatusaccording to claim 1, wherein said second surface is a reflectingsurface.
 3. An image display apparatus according to claim 1, whereinlight rays emitted from said image display device enter said ocularoptical system through said first surface and are led to said observer'seyeball through said third surface.
 4. An image display apparatusaccording to claim 3, wherein said third surface is identical with saidfirst surface.
 5. An image display apparatus according to claim 1,wherein light rays emitted from said image display device enter saidocular optical system through said first surface and are reflected by afourth surface and said second surface in the mentioned order beforebeing led to said observer's eyeball through said third surface.
 6. Animage display apparatus according to claim 5, wherein said fourthsurface is identical with said third surface.
 7. An image displayapparatus according to claim 6, wherein the reflection at said fourthsurface is total reflection.
 8. An image display apparatus according toclaim 5, wherein said fourth surface is identical with said firstsurface.
 9. An image display apparatus according to claim 5, whereinsaid second surface is identical with said first surface.
 10. An imagedisplay apparatus according to claim 1, wherein said light-blockingmember is provided with an aperture.
 11. An image display apparatusaccording to claim 10, which satisfies the following condition:

    Ltanθ+.o slashed./2≦R≦2 (Ltanθ+.o slashed./2)(22)

where R is a distance from an edge of the aperture in saidlight-blocking member to said observer's visual axis in a sectioncontaining a center of said image display device and said observer'svisual axis, .o slashed. is a diameter of an exit pupil of said ocularoptical system, L is an eye relief of said ocular optical system, and θis a half view angle in the section containing the center of said imagedisplay device and said observer's visual axis.
 12. An image displayapparatus according to claim 10, which satisfies the followingcondition:

    Ltanθ+.o slashed./2≦R≦1.2 (Ltanθ+.o slashed./2)(22')

where R is a distance from an edge of the aperture in saidlight-blocking member to said observer's visual axis in a sectioncontaining a center of said image display device and said observer'svisual axis, .o slashed. is a diameter of an exit pupil of said ocularoptical system, L is an eye relief of said ocular optical system, and θis a half view angle in the section containing the center of said imagedisplay device and said observer's visual axis.
 13. An image displayapparatus according to claim 1, wherein said light-blocking member is inclose contact with said ocular optical system.
 14. An image displayapparatus comprising:an image display device that displays an image; anocular optical system that leads the image displayed by said imagedisplay device to an observer's eyeball without forming an intermediatereal image; and a retaining component that retains both said imagedisplay device and said ocular optical system on an observer's head orface, wherein said image display apparatus includes a numerical aperturelimiting member disposed between said image display device and saidocular optical system to limit a numerical aperture of a bundle of lightrays emitted from said image display device, and said ocular opticalsystem has three to four optical surfaces, and an area surrounded bysaid optical surfaces is filled with a transparent medium having arefractive index larger than 1, said optical surfaces including a firstsurface disposed to face said image display device, a second surfacedisposed on an observer's visual axis to face an observer's pupil, and athird surface disposed on said observer's visual axis between saidsecond surface and said observer's pupil.
 15. An image display apparatusaccording to claim 14, wherein said second surface is a reflectingsurface.
 16. An image display apparatus according to claim 14, whereinlight rays emitted from said image display device enter said ocularoptical system through said first surface and are led to said observer'seyeball through said third surface.
 17. An image display apparatusaccording to claim 16, wherein said third surface is identical with saidfirst surface.
 18. An image display apparatus according to claim 14,wherein light rays emitted from said image display device enter saidocular optical system through said first surface and are reflected by afourth surface and said second surface in the mentioned order beforebeing led to said observer's eyeball through said third surface.
 19. Animage display apparatus according to claim 18, wherein said fourthsurface is identical with said third surface.
 20. An image displayapparatus according to claim 19, wherein the reflection at said fourthsurface is total reflection.
 21. An image display apparatus according toclaim 18, wherein said fourth surface is identical with said firstsurface.
 22. An image display apparatus according to claim 18, whereinsaid second surface is identical with said first surface.
 23. An imagedisplay apparatus according to claim 14, wherein said numerical aperturelimiting member limits the numerical aperture of the bundle of lightrays emitted from said image display device such that an angle θ betweena normal to said image display device and a marginal light ray on anobserver's pupil side is not greater than 45°.
 24. An image displayapparatus according to claim 14, wherein said numerical aperturelimiting member limits the numerical aperture of the bundle of lightrays emitted from said image display device such that an angle θ betweena normal to said image display device and a marginal light ray on anobserver's pupil side is not greater than 20°.
 25. An image displayapparatus according to claim 14, wherein said numerical aperturelimiting member is a louver.
 26. An image display apparatus according toclaim 14, wherein said numerical aperture limiting member is an opticalfiber plate.
 27. An image display apparatus according to claim 14,wherein said numerical aperture limiting member is a field selectingglass.
 28. An image display apparatus comprising:an image display devicethat displays an image: an ocular optical system that leads the imagedisplayed by said image display device to an observer's eyeball withoutforming an intermediate real image; and a retaining component thatretains both said image display device and said ocular optical system onan observer's head or face, said image display device being atransmission type liquid crystal display device, said transmission typeliquid crystal display device using light limited in numerical apertureas illuminating light therefor, wherein said ocular optical system hasthree to four optical surfaces, and an area surrounded by said opticalsurfaces is filled with a transparent medium having a refractive indexlarger than 1, said optical surfaces including a first surface disposedto face said image display device, a second surface disposed on anobserver's visual axis to face an observer's pupil, and a third surfacedisposed on said observer's visual axis between said second surface andsaid observer's pupil.
 29. An image display apparatus according to claim28, wherein said second surface is a reflecting surface.
 30. An imagedisplay apparatus according to claim 29, wherein light rays emitted fromsaid image display device enter said ocular optical system through saidfirst surface and are led to said observer's eyeball through said thirdsurface.
 31. An image display apparatus according to claim 30, whereinsaid third surface is identical with said first surface.
 32. An imagedisplay apparatus according to claim 29, wherein light rays emitted fromsaid image display device enter said ocular optical system through saidfirst surface and are reflected by a fourth surface and said secondsurface in the mentioned order before being led to said observer'seyeball through said third surface.
 33. An image display apparatusaccording to claim 32, wherein said fourth surface is identical withsaid third surface.
 34. An image display apparatus according to claim33, wherein the reflection at said fourth surface is total reflection.35. An image display apparatus according to claim 32, wherein saidfourth surface is identical with said first surface.
 36. An imagedisplay apparatus according to claim 32, wherein said second surface isidentical with said first surface.